Method of making solid-state illumination panels with thin and/or flexible sheet forms

ABSTRACT

A method for making a wide-area illumination panel that uses solid-state light sources is disclosed. The method includes providing a back sheet, a front sheet approximately the same size as the back sheet, and a light emitting panel comprising one or more solid-state light sources and having an area smaller than the front sheet. One or more strips of double sided adhesive tape or other adhesive material with a thickness approximately equal to or greater than the total thickness of the light emitting portion of the light emitting panel are also provided. The light emitting panel is placed between the front and back sheets, and the front sheet is attached to the back sheet along one or more perimeter edges using the adhesive tape or the adhesive material, creating a thin and hollow sheet-like structure enclosing at least part of the light emitting panel.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation of application Ser. No. 17/214,843filed on Mar. 27, 2021, which is a continuation of application Ser. No.16/840,382 filed on Apr. 4, 2020. This application also claims priorityfrom U.S. provisional application Ser. No. 62/829,624 filed on Apr. 4,2019, the disclosure of which is incorporated herein by reference in itsentirety.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT

Not Applicable

INCORPORATION-BY-REFERENCE OF MATERIAL SUBMITTED ON A COMPACT DISC

Not Applicable

NOTICE OF MATERIAL SUBJECT TO COPYRIGHT PROTECTION

A portion of the material in this patent document is subject tocopyright protection under the copyright laws of the United States andof other countries. The owner of the copyright rights has no objectionto the facsimile reproduction by anyone of the patent document or thepatent disclosure, as it appears in the United States Patent andTrademark Office publicly available file or records, but otherwisereserves all copyright rights whatsoever. The copyright owner does nothereby waive any of its rights to have this patent document maintainedin secrecy, including without limitation its rights pursuant to 37C.F.R. § 1.14.

BACKGROUND OF THE INVENTION 1. Field of the Invention

The present invention relates to wide-area illumination devicesemploying planar and linear light guides as well as compact solid-statelight emitting devices such as light emitting diodes (LEDs) or laserdiodes. More particularly, this invention relates to wide-area LEDillumination devices such as those employed in lighting panels, lightingluminaires, decorative lights, illuminated panel signs, electronicdisplays, front lights, backlights, backlit display screens, advertisingdisplays, road signs, decorative broad-area lights, as well as to amethod for redistributing light from a variety of light sources in suchdevices. The invention further relates to illumination devices in whichplanar-type or linear light guides are retained in a bent or curvedstate.

2. Description of Background Art

Conventionally, wide-area light emitting devices employ planar lightguides, also commonly referred to as “waveguides”, which are illuminatedfrom one or more edges using Light Emitting Diodes (LEDs) or other typesof compact light sources. The conventional waveguide-based illuminationsystems may exhibit certain limitations such as difficulty toefficiently couple, decouple and/or distribute light, particularlywithin a thin and flexible form factor. Additionally, configuring thewaveguide-based illumination systems for a desired spatial and/orangular emission distribution and/or uniformity of the emission may beassociated with optical losses and lead to energy waste and suboptimalperformance.

U.S. patent Ser. No. 10/030,846 (the '846 patent) and US20170045666 (the'666 Publication), the disclosure of which is incorporated herein byreference in its entirety, disclose face-lit waveguide illuminationsystems formed by a planar waveguide and optical coupling elementsattached to a face of the waveguide. U.S. Pat. Nos. 9,256,007, 9,097,826(the '826 Pat.), 8290318, and U.S. Patent Applications Publication No.US20140140091, the disclosure of which is incorporated herein byreference in its entirety, disclose various configurations of waveguides(light guides) and light deflecting/light extraction elements.

U.S. Patent Applications Publication No. US20180348423 (the '423Publication), the disclosure of which is incorporated herein byreference in its entirety, discloses various configurations of steppedlight guides and light guide illumination systems, as well as differentarrangements of solid-state light sources and light extraction features.U.S. Patents No. D777972, D776331, D799738, D814101, D824085, D824086,and D824087, the disclosure of which is incorporated herein by referencein its entirety, disclose exemplary light emitting patterns associatedwith light emitting sheet-form structures. U.S. Patents No. U.S. Ser.No. 10/267,972 (the '972 Patent), D825097 (the '097 Patent), D866841(the '841 Patent), and D845538 (the '538 Patent), the disclosure ofwhich is incorporated herein by reference in its entirety, disclosesshaped light guide illumination devices.

It is noted that, where a definition or use of a term in a reference,which is incorporated by reference herein is inconsistent or contrary tothe definition of that term provided herein, the definition of that termprovided herein applies and the definition of that term in the referencedoes not apply.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWING(S)

The invention will be more fully understood by reference to thefollowing drawings which are for illustrative purposes only:

FIG. 1 is a schematic perspective view and raytracing of a wide-areasolid-state light guide illumination system, according to at least oneembodiment of the present invention.

FIG. 2 is a schematic view of an adjacent pair of spaced-apart lightextraction features formed on a surface of a light guiding sheet,illustrating exemplary definitions of a spacing distance and aseparation distance, according to at least one embodiment of the presentinvention.

FIG. 3 is a schematic view of contacting light extraction features,according to at least one embodiment of the present invention.

FIG. 4 is a schematic view of substantially overlapping light extractionfeatures, according to at least one embodiment of the present invention.

FIG. 5 is a schematic perspective view of a wide-area light guideillumination system having a segmented light extraction pattern,according to at least one embodiment of the present invention.

FIG. 6 is a schematic plan view of a wide-area light guide illuminationsystem, showing an exemplary alternative arrangement of individual lightextraction areas and separation areas of a light extraction pattern,according to at least one embodiment of the present invention.

FIG. 7 is a schematic view of portion of a wide-area light guideillumination system, showing a two-dimensional array of hexagonal lightextracting sections distributed over an area of a light guiding sheet,according to at least one embodiment of the present invention.

FIG. 8 is a schematic view showing an exemplary variable-densitydistribution pattern of light extraction features for a portion of awide-area light guide illumination system, according to at least oneembodiment of the present invention.

FIG. 9 is a schematic section view of a portion of a wide-area lightguide illumination system, showing an irregularly shaped lightextraction feature formed on a surface of a light guide, according to atleast one embodiment of the present invention.

FIG. 10 is a schematic surface profile illustrating exemplarymeasurements of a surface waviness and a contact angle, according to atleast one embodiment of the present invention.

FIG. 11 is a schematic graph showing a measured dependency of theopacity of a light guiding sheet on spacing between light extractionfeatures, according to at least one embodiment of the present invention.

FIG. 12 is a schematic view of a surface portion of a light guidingsheet having a plurality of light extraction features having variousshapes and sizes, according to at least one embodiment of the presentinvention.

FIG. 13 is a schematic section view and raytracing of a portion of awide-area light guide illumination system, showing light extractionfeatures including at least two layers of different materials, accordingto at least one embodiment of the present invention.

FIG. 14 is a schematic section view and raytracing of a portion of afront light including light blocking areas provided on a transparentsubstrate, according to at least one embodiment of the presentinvention.

FIG. 15 is a schematic section view of a portion of a wide-area lightguide illumination system having multiple stacked light guiding sheets,each including light extraction patterns on both opposing broad-areasurfaces, according to at least one embodiment of the present invention.

FIG. 16 is a schematic section view of a portion of a wide-area lightguide illumination system having multiple stacked light guiding sheets,showing light diffusing sheets positioned between the light guidingsheets, according to at least one embodiment of the present invention.

FIG. 17 is a schematic section view of a portion of a wide-area lightguide illumination system having different types of light extractionfeatures formed in opposing broad-area surfaces of a light guidingsheet, according to at least one embodiment of the present invention.

FIG. 18 is a calculated polar luminous intensity distribution graph foran exemplary configuration of light extraction features, showing asymmetrical Lambertian emission from opposing broad-area surfaces of aplanar light guide with equal top/bottom light output, according to atleast one embodiment of the present invention.

FIG. 19 is a calculated polar luminous intensity distribution graph foran alternative exemplary configuration of light extraction features,showing a Lambertian emission from opposing broad-area surfaces of aplanar light guide with unequal top/bottom light output, according to atleast one embodiment of the present invention.

FIG. 20 is a calculated polar luminous intensity distribution graph fora further alternative exemplary configuration of light extractionfeatures, showing different types of “batwing” emission from opposingtop and bottom broad-area surfaces of a planar light guide, according toat least one embodiment of the present invention.

FIG. 21 is a calculated polar luminous intensity distribution graph fora yet further alternative exemplary configuration of light extractionfeatures, showing a directional emission from a broad-area surface of aplanar light guide, according to at least one embodiment of the presentinvention.

FIG. 22 is a measured polar luminous intensity distribution graph for anexemplary configuration of a wide-area illumination system, showing aquasi-Lambertian emission from opposing top and bottom broad-areasurfaces of a planar light guide, according to at least one embodimentof the present invention.

FIG. 23 is a measured polar luminous intensity distribution graph for anexemplary configuration of a wide-area illumination system, showing anear-Lambertian emission from a single broad-area surface of a planarlight guide, according to at least one embodiment of the presentinvention.

FIG. 24 is a photograph of an ordered pattern of semi-opaque lightextraction features printed on a surface of a light guiding sheet,according to at least one embodiment of the present invention.

FIG. 25 is a photograph of a random pattern of semi-opaque lightextraction features printed on a surface of a light guiding sheet with arelatively high areal density, according to at least one embodiment ofthe present invention.

FIG. 26 is a photograph of an ordered pattern of optically clear,elongated light extraction features printed on a surface of a lightguiding sheet, according to at least one embodiment of the presentinvention.

FIG. 27 is a photograph of a lower-density ordered pattern of opticallyclear, elongated light extraction features printed on a surface of alight guiding sheet, according to at least one embodiment of the presentinvention.

FIG. 28 is a microphotograph of an individual semi-opaque lightextraction feature printed on a surface of a light guiding sheet andhaving a round or near-round shape at the base, according to at leastone embodiment of the present invention.

FIG. 29 is a microphotograph of an individual semi-opaque lightextraction feature printed on a surface of a light guiding sheet andhaving an irregular elongated shape at the base, according to at leastone embodiment of the present invention.

FIG. 30 is a series of photographs of portions of a surface of a planarlight guide, showing different patterns and distribution densities ofmicrodots of a light scattering material, according to at least oneembodiment of the present invention.

FIG. 31 is a three-dimensional composite image of an exemplaryindividual microdot of a light scattering material on a surface of aplanar light guide, according to at least one embodiment of the presentinvention.

FIG. 32 is a graph showing a measured cross sectional height profile ofthe microdot depicted in FIG. 31 .

FIG. 33 is a schematic section view and raytracing of a portion of awide-area light guide illumination system in a backlight configuration,showing light extraction features having different structures andcomposition, according to at least one embodiment of the presentinvention.

FIG. 34 is a schematic top plan view of a portion of a light emittingsurface of a wide-area light guide illumination system, showingdifferent types of light extraction features, according to at least oneembodiment of the present invention.

FIG. 35 is a schematic front view of a wide-area light guideillumination system in the form of an edge-lit illuminated sign havingletter images emitting light in different colors, showing differenttypes of light extraction features, according to at least one embodimentof the present invention.

FIG. 36 is a schematic section view of a wide-area light guideillumination system having light deflective elements formed from anadhesive ink material and configured for bonding e reflective sheet to alight guiding sheet.

FIG. 37 is a schematic view of an apparatus for forming light extractionfeatures on a surface of a light guide using radiation-curable ink,according to at least one embodiment of the present invention.

FIG. 38 is a graph schematically showing dependencies of the diameterand thickness of a light extraction feature from the time between adeposition of a microdrop of a UV-curable ink to the surface of a lightguiding substrate and beginning of irradiation of the microdrop by a UVlight source, according to at least one embodiment of the presentinvention.

FIG. 39 is a schematic view illustrating a method of forming a patternof light extraction features on a surface of a light guide usingradiation-curable ink and a material transfer film, according to atleast one embodiment of the present invention.

FIG. 40 is a schematic view of a method of forming light extractionpatterned areas on a surface of a light guide using radiation-curableink and area fill, according to at least one embodiment of the presentinvention.

FIG. 41 is a photograph of an exemplary randomized pattern of lightextraction features printed on a surface of a light guiding substrateusing UV curable light scattering ink and an instant cure printingprocess.

FIG. 42 is a schematic front view of a wide-area light guideillumination system implemented in the form of an edge-lit illuminatedsign having a light emitting area patterned with a variable density oflight extraction features, according to at least one embodiment of thepresent invention.

FIG. 43 is a schematic exploded perspective view of a wide-area lightguide illumination system having a layered structure including back andfront sheets, according to at least one embodiment of the presentinvention.

FIG. 44 is a schematic exploded perspective view of a wide-area lightguide illumination system having a layered structure including back andfront sheets, showing reflective areas formed in the back sheet,according to at least one embodiment of the present invention.

FIG. 45 is a schematic perspective view of a wide-area light guideillumination system, showing a front sheet formed into a sleeve andcovering a light guiding sheet, according to at least one embodiment ofthe present invention.

FIG. 46 is a schematic perspective view of a wide-area light guideillumination system, showing a light guide-covering sleeve in adifferent orientation, according to at least one embodiment of thepresent invention.

FIG. 47 is a schematic perspective view of a wide-area light guideillumination system, showing a sleeve covering a rectangular lightguiding sheet from three edges, according to at least one embodiment ofthe present invention.

FIG. 48 is a schematic section view of a wide-area light guideillumination system, showing a sleeve completely covering a broad-areasurface and a pair of opposing edges of a light guide, according to atleast one embodiment of the present invention.

FIG. 49 is a schematic section view of a wide-area light guideillumination system, showing a sleeve which at least partially enclosesa light guiding sheet and has side flaps bonded to a back sheet,according to at least one embodiment of the present invention.

FIG. 50 is a schematic section view of a wide-area light guideillumination system, showing a sleeve enclosing a light guiding sheetand formed by a frond sheet and a back sheet bonded together, accordingto at least one embodiment of the present invention.

FIG. 51 is a schematic section view of a wide-area light guideillumination system, showing a sleeve enclosing a light guiding sheetand formed by a frond sheet and a back sheet joined together at edges,according to at least one embodiment of the present invention.

FIG. 52 is a schematic section view of a portion of a wide-area lightguide illumination system, showing structural members fastened to alight guiding sheet, according to at least one embodiment of the presentinvention.

FIG. 53 is a schematic section view of a wide-area light guideillumination system having a light guiding sheet folded at amid-section, according to at least one embodiment of the presentinvention.

FIG. 54 is a schematic perspective view of a linear light guideillumination system having a light guiding rod patterned usingarea-distributed light extraction features, according to at least oneembodiment of the present invention.

FIG. 55 is a schematic section view of a linear light guide illuminationsystem in which a light guiding rod has planar surfaces and a curvedsurface patterned using area-distributed light extraction features,according to at least one embodiment of the present invention.

FIG. 56 is a schematic section view of a linear light guide illuminationsystem in which a light guiding rod has a rectangular cross-section,according to at least one embodiment of the present invention.

FIG. 57 is a schematic section view of a linear light guide illuminationsystem in which a light guiding rod has a corrugated viewable surface,according to at least one embodiment of the present invention.

FIG. 58 is a schematic section view of a linear light guide illuminationsystem in which a light guiding rod has a round cross-section, accordingto at least one embodiment of the present invention.

FIG. 59 is a schematic section view of a linear light guide illuminationsystem in which a light guiding rod has a half round cross-section,according to at least one embodiment of the present invention.

FIG. 60 is a schematic section view of a linear light guide illuminationsystem incorporating a sleeve which encloses a round light guiding rod,according to at least one embodiment of the present invention.

FIG. 61 is a schematic section view of a linear light guide illuminationsystem incorporating a sleeve which encloses a square guiding rod,according to at least one embodiment of the present invention.

FIG. 62 is a schematic perspective view of a wide-area light guideillumination system formed by a series of linear light guideillumination systems positioned side by side and arranged into a planararray, according to at least one embodiment of the present invention.

FIG. 63 is a schematic view of a wide-area light guide illuminationsystem having planar and curved light guiding rods, according to atleast one embodiment of the present invention.

FIG. 64 is a schematic perspective view of a wide-area light guideillumination system having a two-dimensional array of light guiding rodsand LEDs, according to at least one embodiment of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring more specifically to the drawings, for illustrative purposes,the present invention is embodied in the systems generally shown in thepreceding figures. It will be appreciated that the systems may vary asto configuration and as to details of the parts without departing fromthe basic concepts as disclosed herein. Furthermore, elements orlimitations represented in one embodiment as taught herein areapplicable without limitation to other embodiments taught herein and/orin patents and patent applications incorporated by reference herein, andin combination with those embodiments and what is known in the art.

A wide range of applications exist for the present invention in relationto the collection and distribution of electromagnetic radiant energy,such as light, in a broad spectrum or any suitable spectral bands ordomains. Therefore, for the sake of simplicity of expression, withoutlimiting generality of this invention, the term “light” will be usedherein although the general terms “electromagnetic energy”,“electromagnetic radiation”, “radiant energy” or exemplary terms like“visible light”, “infrared light”, or “ultraviolet light” would also beappropriate.

Furthermore, many applications exist for the present invention inrelation to distributing light by means of a planar optical light guidewhich hereinafter may also be referenced to as a waveguide. The planaroptical light guide (planar waveguide) refers to a broad class ofobjects employing an optically transmissive material confined betweentwo opposing broad-area surfaces that extend substantially parallel toeach other. The term “substantially parallel” generally includes caseswhen the opposing surfaces are parallel within a reasonable accuracy. Italso includes cases when the body of the material defined by thebroad-area surfaces has a slightly tapered shape or has a slightlyvarying thickness across the surface. It yet further includes cases whena generally planar body of the light guide includes limited areas whereits thickness is different compared to the rest of the light guide.

According to a preferred embodiment of the present invention, the planarlight guide may be exemplified by a transparent plate, sheet, slab,panel, pane, light-transmitting substrate or any suitable sheet-form ofan optically transmissive material, including those having filmthicknesses and rigid or flexible sheet forms. This invention is alsoapplicable to any two-dimensional shape variations of the sheet forms,including but not limited to a square, rectangle, polygon, circle,strip, freeform, or any combination therein. This invention is furtherapplicable to any three-dimensional shapes that can be obtained bybending the sheet forms accordingly, including but not limited tocylindrical or partial cylindrical shapes, conical shapes, corrugatedshapes, and the like. Opposite ends or edges of such three-dimensionalshapes may be also be connected together to form a continuous surface orsurfaces. For example, a strip of a light guiding strip material can bebent and its edges connected so as to form a loop. In a further example,a strip of a light guiding strip material can be folded at a mid-sectionsuch that its opposite edges come in contact with each other and form acombined edge of the material.

It is also noted that terms such as “top”, “bottom”, “side”, “front”,“back”, “left”, and “right” and similar directional terms may be usedherein with reference to the orientation of the Figures being describedand should not be regarded as limiting this invention in any way. Itshould be understood that different elements of embodiments of thepresent invention can be positioned in a number of differentorientations without departing from the scope of the present invention.In the context of the description of a planar light guide and itselements, the term “top” is being generally used to refer to a primarylight emitting side of the light guide and the term “bottom” is beinggenerally used to refer to the opposite side (which may be emitting ornon-emitting) for the sake of convenience of description and not in alimiting sense.

Any numerical range recited herein is intended to include all sub-rangessubsumed therein. For example, a range of 1 to 10 is intended to includeall sub-ranges between and including the recited minimum value of 1 andthe recited maximum value of 10, that is, having a minimum value equalto or greater than 1 and a maximum value of equal to or less than 10,such as, for example, 3 to 6 or 2.5 to 8.5. Any maximum numericallimitation recited herein is intended to include all lower numericallimitations subsumed therein and any minimum numerical limitationrecited in this specification is intended to include all highernumerical limitations subsumed therein. Accordingly, Applicants reservethe right to amend this specification, including the claims, toexpressly recite any sub-range subsumed within the ranges expresslyrecited herein. All such ranges are intended to be inherently describedin this specification such that amending to expressly recite any suchsubranges would comply with the requirements of 35 U.S.C. § 112, firstparagraph, and 35 U.S.C. § 132(a). Also, unless otherwise indicated, allnumerical parameters are to be understood as being prefaced and modifiedin all instances by the term “about”, in which the numerical parameterspossess the inherent variability characteristic of the underlyingmeasurement techniques used to determine the numerical value of theparameter.

According to some preferred embodiments of the present invention, thereis provided an illumination system employing an optical light guideexemplified by an optically transmissive, broad-area sheet or panel,which may also hereinafter be referred to as a “light guiding sheet”,“light guiding panel” or “LGP”. The LGP is made from a material whichhas a refractive index greater than that of the outside medium and iscapable of guiding light within the panel by means of a Total InternalReflection (TIR) from its opposing broad-area surfaces, provided thatthe internal incidence angles onto either of the surfaces are greaterthan a critical angle of TIR characterizing the broad-area surfaces.

For the purpose of this discussion, the term “incidence angle” of alight ray in relation to a surface generally refers to an angle thatsuch ray makes with respect to a normal to the surface. It will beappreciated by those skilled in the art of optics that, when referringto light or other forms of electromagnetic waves passing through aboundary formed between two different refractive media, such as air andglass, for example, the ratio of the sines of the angles of incidenceand of refraction is a constant that depends on the ratio of refractiveindices of the media (the Snell's law of refraction). The followingrelationship can describe a light bending property of an interfacebetween two refractive media: n₁ sin ϕ₁=n_(R) sin ϕ_(R), where n₁ andn_(R) are the respective refractive indices of the materials forming theoptical interface and ϕ₁ and ϕ_(R) are the angle of incidence and theangle of refraction, respectively. It will be further appreciated thatsuch optical interface can also be characterized by a critical TIR anglewhich is the value of ϕ₁ for which ϕ_(R) equals 90°. Accordingly, for asurface characterized by a stepped drop in refractive index along thepropagation path of a ray, the incidence angle may be less than, equalto, or greater than the TIR angle at the given surface.

A TIR angle ϕ_(TIR) can be found from the following expression:

$\begin{matrix}{\phi_{TIR} = {{\arcsin( {{\frac{n_{R}}{n_{I}} \cdot \sin}90{^\circ}} )} = {\arcsin( \frac{n_{R}}{n_{I}} )}}} & ( {{Equation}1} )\end{matrix}$

In an exemplary case of the interface between glass with the reflectiveindex n₁ of about 1.51 and air with n_(R) of about 1, ϕ_(TIR) isapproximately equal to 41.5°. It will be appreciated that, once light isinput into the LGP and its propagation angles permit for TIR to occur atLGP's longitudinal walls, the light becomes trapped in the LGP and canpropagate considerable distances until it is extracted, absorbed orreaches an edge of the panel, for example.

The present invention will now be described by way of example withreference to the accompanying drawings.

FIG. 1 schematically depicts an embodiment of a wide-area light guideillumination system 900 in accordance with the invention. Light guideillumination system 900 includes a generally planar light guide 800 thatis formed by a substantially planar base sheet 10 (light guiding sheet)of an optically transmissive material. Light guide 800 may also have aplurality of secondary sheets of an optically transmissive material (notshown) attached to the base sheet 10 (such as, for example, sheets 20described in reference to FIGS. 1-2 of the '423 Publication). Theorientation of planar light guide 800 and its components inthree-dimensional space may be conveniently described using orthogonalreference axes X, Y, and Z (see FIG. 1 ) which also define orthogonalreference planes XY, XZ, and YZ.

Sheet 10 has a rectangular configuration and is defined by opposingbroad-area surfaces 11 and 12 and four edge surfaces 13, 14, 15 and 16.Surfaces 11 and 12 represent major surfaces of planar light guide 800that are configured to guiding light using TIR. Surfaces 11 and 12extend parallel to each other. They also extend broadly bothlongitudinally and laterally along the X and Y axes so as to form aplanar sheet form that is parallel to the XY plane. Opposing edgesurfaces 13, 14 are parallel to each other, extending parallel to the XZplane, and opposing edge surfaces 15, 16 are likewise parallel to eachother, extending parallel to the YZ plane. Sheet 10 has a non-zerothickness which may be conventionally measured along the Z axis orcoordinate.

Sheet 10 is preferably formed from a highly transmissive, soliddielectric material and is configured to guide light both longitudinallyand laterally using optical transmission through the material and TIRfrom opposing surfaces 11 and 12. Surfaces 11 and 12 are preferablyoptically smooth and polished to high gloss. Edge surfaces 13, 14, 15and 16 may also be polished and configured for reflecting light withhigh efficiency using TIR. One or more edge surfaces 13, 14, 15 and 16may also be covered with a specularly reflective mirror or a diffusereflector. For example, any of the edge surfaces may be coated with ametallic layer (e.g., aluminized or silvered). In another example,strips of highly reflective material, such as a metallized film or foilmay be applied to any of the edge surfaces 13, 14, 15 and 16. Anothersuitable exemplary type of the reflective materials may also be awhite-color, light diffusing tape.

Suitable materials for making sheet 10 may include various dielectricmaterials in the form of a wide-area, highly transparent sheet or film.Materials that may be particularly suited for making sheet 10 includebut are not limited to water-clear (low-iron) glass, Poly(methylmethacrylate) (PMMA or acrylic), polycarbonate (PC), Styrene methylmethacrylate (SMMA), Polystyrene (PS), Polyethylene Terephthalate Glycol(PETG), methacrylate styrene copolymer (MS), cured urethane, polyester,silicone, and the like.

Sheet 10 has a length L₁₀ and a width W₁₀, which can be approximatelyequal to length L₁₀ or considerably less than length L₁₀. According todifferent embodiments, width W₁₀ can be less than length L₁₀ by at least1.5 times, at least 2 times, at least 3 times, at least 5 times, and atleast 10 times. The thickness of sheet 10 can be made sufficiently lowto make it flexible. According to one embodiment, the thickness offlexible sheet 10 can be in the range of 0.3 mm to 2.5 mm, and morepreferably in the range from 0.5 mm to 1.5 mm so that the sheet could beflexed and handled with relative ease without breaking or affecting itsstructural integrity.

Light guide illumination system 900 further includes a plurality ofcompact solid-state light sources exemplified by LEDs 2. LEDs 2 areprovided in four linear arrays or strips where each array or strip isconfigured to illuminate the respective edge of sheet 10 (e.g., edgesurfaces 13, 14, 15 and 16). In other words, all of the edges of sheet10 can be configured as light input edges. Within each array or strip,LEDs 2 are positioned adjacent and optically coupled to the respectiveedge surface such that the amount of light that is not coupled to lightguide 800 (light spillage) is minimized. LEDs 2 may be exemplified bytop-emitting or side-emitting LED packages which may be conventionallyarranged on a rigid or flexible strip. The LED strip may conventionallyinclude a heat-spreading printed circuit board (PCB) or flexible film orsubstrate.

Surface 11 of sheet 10 includes a plurality of discrete light extractionfeatures 8 forming a two-dimensional light extraction pattern 101 andconfigured to extract light from light guide 800 such that the extractedlight is emitted from the entire two-dimensional area of lightextraction pattern 101. According to one embodiment, light extractionpattern 101 may occupy substantially the entire exposed area of lightguide 800 (e.g., extending all the way longitudinally between opposingedge surfaces 13 and 14 and laterally between opposing edge surfaces 15and 16).

According to one embodiment, light extraction features 8 are formed onlyin surface 11 while opposite surface 12 can be substantially free fromlight extraction features 8. According to one embodiments, lightextraction features 8 are formed only in surface 12 while surface 11 canbe substantially free from light extraction features 8. According to oneembodiments, light extraction features 8 are formed in both opposingsurfaces 11 and 12. The arrangement or spatial distribution of lightextraction features 8 formed in surface 11 may be the same or differentthan the arrangement or spatial distribution of light extractionfeatures 8 formed in surface 12.

While some of the paragraphs below may describe embodiments ofillumination system 900 primarily referring to light extraction features8 being formed in surface 11, it should be understood that thisinvention is not limited to this and that the same description can beapplied to the cases when light extraction features 8 are formed insurface 12 or in both surfaces 11 and 12. Furthermore, it should beunderstood that these embodiments are amenable to various modificationsand alternative forms, for example, in which light extraction features 8are formed in other surfaces that are parallel or near-parallel tosurfaces 11 and 12, especially when light guide 800 includes two or morelayer of optical transparent materials. According to some embodiments,light guide 800 may be formed by two, three or more sheets of opticallytransmissive materials that are attached (and optionally bonded) to eachother. At least some of light extraction features 8 may be formed in oron the inside broad-area surface of one or all of the sheets, so that atleast the inner light extraction features 8 can be made embedded intothe material of light guide 800 when the sheets are bonded together.According to one embodiment, such multiple sheets may be bonded to eachother at their edges or selected locations of their surfaces.

In operation, an exemplary light ray 133 emitted by one of LEDs 2optically coupled to light input edge surface 13 is propagated withinsheet 10 in a waveguide mode until it is extracted by one of lightextraction features 8 and is directed out and away from light guide 800.Depending on the location, configuration and optical properties ofextraction features 8, as well as probability, ray 133 may exit fromeither surface 11 or 12. Depending on the same factors, ray 133 may exitfrom surface 11 or 12 at a right angle or at an oblique angle withrespect to the surface plane. A small portion of light emitted by theLED may also be allowed to exit from one or more edges of light guide800.

According to one embodiment, light extraction features 8 may beconfigured to direct at least a portion of the deflected light back intothe body of sheet 10 at angles permitting for continued propagation ofthe light through sheet 10 in a waveguide mode and further configured toextract the respective portion of light at a different location of thesurface of sheet 10. Redirecting a portion of the deflected light backinto light guiding sheet 10 may be critical at least for someembodiments of system 900 configured for enhanced mixing of light withinlight guide 800. In other words, light extraction features 8 may beconfigured to randomize and mix light rays within the body of sheet 10such that a light ray may be deflected by one or more light extractionfeatures 8 while propagating in sheet 10 and finally extracted at adifferent location by one of other light extraction features 8.

According to one embodiment, sheet 10 may be configured to transmitlight in a transverse direction through spaces between light extractionfeatures 8 such that light guide 800 (or at least large portions of it)has a transparent appearance when in a non-illuminated state. Accordingto various embodiments, one or more lighting diffuser sheets may bepositioned on either one or both light-emitting sides of sheet 10(surfaces 11 and 12). The lighting diffusers may be of a transmissivetype (e.g., a translucent sheet or a textured sheet of opticallytransparent material). At least one of the lighting diffusers may be ofa reflective type (e.g., a mirrored sheet or a diffusely reflectivesheet). The diffuser sheet(s) may be laid onto either one of surfaces 11and 12, preferably without disrupting TIR in sheet 10. Suitableexemplary arrangements of the lighting diffusers with respect to sheet10 may include but are not limited to the following exemplarycombinations: an optically transmissive diffuser is provided on the sideof surface 11, an optically transmissive diffuser is provided on theside of surface 12, a reflective diffuser is provided on the side ofsurface 11, a reflective diffuser is provided on the side of surface 12,an optically transmissive diffuser is provided on the side of surface 11and a reflective diffuser is provided on the side of surface 12, anoptically transmissive diffuser is provided on the side of surface 12and a reflective diffuser is provided on the side of surface 11, and afirst optically transmissive diffuser is provided on the side of surface12 and a second optically transmissive diffuser is provided on the sideof surface 11.

According to one embodiment, light extraction pattern 101 may have auniform average areal density or surface coverage with a randomizedspacing between individual light extraction features 8, for example, asschematically illustrated in FIG. 1 . The spacing may be randomized, forexample, such that adjacent individual light extraction features 8 arespaced from each other by spacing distances that deviate from an averagespacing within a sampling area by no less than a minimum spacingdistance and no more than a maximum spacing distance.

A spacing distance (SPD) may be ordinarily defined as a distance betweengeometrical centers of individual light extraction features 8, e.g., asschematically illustrated in FIG. 2 . According to differentembodiments, a minimum spacing distance SPD_(MIN) characterizing aparticular sampling area of light extraction pattern 101 may be 0.1,0.25, 0.5, 0.75, or 0.9 times an average spacing distance SPD_(AVG)characterizing the same sampling area. According to differentembodiments, a maximum spacing distance SPD_(MAX) characterizing aparticular sampling area of light extraction pattern 101 may be 1.2,1.5, 2, 2.5, 3, 5 or 10 times the average spacing distance SPD_(AVG).

A separation distance (SED) in relation to a pair of adjacent individuallight extraction features 8 may be defined as the shortest distanceconnecting the respective outlines or boundaries of such adjacent lightextraction features (FIG. 2 ). In an exemplary case of two adjacentlight extraction features 8, each having a round outline or aperturewith a diameter d, separation distance SED may be defined as the spacingdistance SPD minus diameter d. Accordingly, depending on the values ofthe spacing distance and diameter d, the separation distance may havenegative and positive values, and may also be zero when SPD=d.

The definition of separation distance SED may also be generalized to thecases where light extraction features 8 have shapes other than round.For example, when light extraction features 8 has an elongated orirregular shape, an average size or diameter of light extractionfeatures 8 may be used in place of diameter d to define separationdistance SED. The average diameter or size of individual lightextraction feature 8 may be defined as an average length of diametersmeasured at predefined angular intervals around a centroid of the shaperepresenting such light extraction feature 8. For example, the angularintervals can be 1°, 2°, 5°, 10°, 20°, 30° or 45°.

According to one embodiment, separation distance SED characterizing apair of adjacent light extraction features 8 having round or non-roundshapes may be considered negative when these adjacent light extractionfeatures 8 (or their apertures or outlines) substantially overlap (FIG.4 ), zero when they (or their apertures or outlines) are disposed incontact or extremely close to each other (FIG. 3 ), and positive whentheir outlines or apertures neither overlap nor contact each other (FIG.2 ).

According to one embodiment, separation distance SED between at leastsome adjacent light extraction features 8 is negative, e.g., lightextraction features 8 are substantially overlapping. According todifferent embodiments, the overlapping light extraction features 8 mayoverlap by 10% or more, 20% or more, 30% or more, 50% or more, 75% ormore, or 80% or more. According to an aspect of the embodiments in whichtwo or more light extraction features 8 overlap, such overlapping lightextraction features 8 may be cumulatively considered a larger singlelight extraction feature 8.

According to one embodiment, at least some light extraction features 8may be formed by a single cured drop of light scattering ink using aninkjet printing process. The individual drops of light scattering inkmay ordinarily have diameters that are much less than a millimeter (andpreferably even less than 100 m) and may thus be referred to asmicrodrops. A fully or partially cured microdrop of light scattering inkformed on a surface of light guide 800 may be referred to as a microdot(e.g., a microscopic-scale solid surface structure that can scatter,reflect or deflect light).

According to one embodiment, individual light extraction features 8 maybe formed by depositing two or more microdrops of a light scattering inkto the same location of surface 11. According to one embodiment,individual light extraction features 8 may be formed by depositing twoor more microdrops approximately to the same location but with a slightoffset with respect to each other. According to one embodiment, it maybe preferred that each microdrop deposited to surface 11 is at leastpartially or completely cured to a high-viscosity or substantially solidstate before depositing a next microdrop on top of it or adjacent to it.The droplet deposition process can be repeated to gradually build up aprescribed thickness and/or size of the respective light extractionfeature 8 in stepped increments based on the volume of individualdroplets. Such stacked or overlapping microdots forming individual lightextraction features 8 may ordinarily have a total volume that is a wholemultiple of the volume of the individual microdrops. For example,individual microdrops having a volume of 4 picoliters may form largermicrodots or light extraction features 8 having volumes of 8 picoliters,12 picoliters, 20 picoliters, 40 picoliters and so on.

According to one embodiment, separation distance SED between at leastsome adjacent light extraction features 8 is zero or near zero. In otherwords, the light extraction features 8 are contacting each other ortheir apertures are very close to each other or have a very smalloverlap (e.g., within less than 10% of the average diameter of each ofthe adjacent light extraction features 8).

According to different embodiments, separation distance SED between atleast some adjacent light extraction features 8 is greater than 25%,greater than 50%, greater than 100%, greater than 150%, or equal to orgreater than 200% of diameter d or an average diameter characterizingthe light extraction features 8. According to different embodiments,separation distance SED between at least some adjacent light extractionfeatures 8 is less than 0.9, less than 0.75, less than 0.5, or less than30% of diameter d (in case of round-apertures) or the average diameter(in case of non-round-apertures) characterizing light extractionfeatures 8.

For the purpose of measuring average values of the spacing distances SPDand/or separation distances SED, a suitable sampling area may be definedas a relatively small-size area of surface 11, at a particular locationof the surface, which includes at least 100 individual light extractionfeatures 8. According to different embodiments, an average separationdistance SED_(AVG) between adjacent light extraction features 8 withinthe sampling area may be greater than 25%, greater than 50%, greaterthan 100%, greater than 150%, or equal to or greater than 200% ofdiameter d or the average diameter characterizing light extractionfeatures 8. According to different embodiments, an average separationdistance between adjacent light extraction features 8 within thesampling area is less than 0.9, less than 0.75, less than 0.5, or lessthan 30% of diameter d (in case of round-apertures) or the averagediameter (in case of non-round-apertures) characterizing lightextraction features 8.

According to one embodiment, light extraction features 8 may be formedby repeatedly depositing a number of individual droplets with a slightoffset from each other. The offset can be selected to be less than theprevalent diameter of the individual microdots formed by each droplet,creating at least partial overlap for the resulting micro- ormacro-dots. According to one embodiment, the direction of the offset canbe maintained for depositing a series of individual droplets such that asingle elongated light extraction feature 8 in the form of a straightline can be formed. According to one embodiment, the direction of theoffset can be varied so as to produce continuous curved lines or acombination of straight and curved lines or line segments. The width ofthe lines or line segments produced by this method may be controlled,for example, by the volume and viscosity of each droplet, additivesaffecting surface tension, wettability of surface 11, temperature of theink and/or the substrate (sheet 10), number of microdrops, amount of theoffset, as well as depositing some of the microdots with a perpendicularoffset from the intended center of the line or line segment. Multiplestraight and/or curved lines may be branched at one or multiplelocations, e.g., to produce a tree-like structure.

According to one embodiment, printed light extraction features 8 may beformed by depositing a large number of microdrops (e.g., of a UV-curableink) that overlap on one another and cover a two-dimensional area whichis much larger than the area of the individual micro drops. Suchtwo-dimensional areas may have different regular or irregular shapes.Examples of the regular shapes include but are not limited to circular,oval, linear, square, rectangular, triangular, hexagonal, octagonal, andthe like. The shapes formed by a two-dimensional pattern of overlappingmicro dots may have rounded corners (e.g., rounded-corner squares,triangles or rectangles). Overlapping printed microdrops forming lightextraction features 8 may also be arranged into various geometricalpatterns, indicia, letters or images.

According to one embodiment, the printed shapes, geometrical patterns,letters or images may have a solid fill (e.g., where the printed inkmaterial completely covers the area of the respective shapes,geometrical patterns, letters or images). According to one embodiment,the printed shapes, geometrical patterns, letters or images may have apartial fill (e.g. with gaps in a solid fill) with various areacoverage. For example, the area coverage in the partial fill can be 10%,30%, 50%, 75%, 90%, and so on.

According to one embodiment, light extraction features 8 may be arrangedinto various indicia, patterns, letters or images in the form of adot-pattern fill. This may be particularly critical for cases where theareas of the respective indicia, patterns, letters or images occupies asignificant fraction of the area of sheet 10 (e.g., greater than 10%,greater than 20%, greater than 30%, and greater than or equal to 50%)and where the rate of light extraction caused by a solid fill may be toohigh such that some portions of the illuminated indicia, patterns,letters or images may appear significantly darker than others. Theindividual dots of the dot-pattern fill may be spaced apart from eachother by distances that are constant or decrease with a distance from alight input edge or edges (e.g., edges/edge surfaces 13 and 14 of sheet10). According to one embodiment, at least some dots (e.g., those whichare farthest from the light input edge) may partially overlap with oneanother and may further form clusters of overlapping dots.

Light extraction features 8 may include any suitable two- orthree-dimensional optical elements or surface features configured forintercepting and extracting light from sheet 10. Light extractionfeatures 8 may be configured to extract light by means of scattering,reflection, refraction, deflection, diffraction, absorption (with thesubsequent re-emission), or any combination thereof.

Light extraction features 8 may be further configured to extract lightwhile changing one or more properties of light. Exemplary properties oflight that may be changed by light extraction features 8 include but arenot limited to a wavelength, polarization, spectral distribution,angular and/or spatial distribution, and dispersion. For example, eachlight extraction features 8 may include a color pigment that receiveswhite color and either filters out certain wavelengths or converts thereceived light to a different color. For example, each light extractionfeatures 8 may include a color pigment that is configured for convertinga white color to a different color, e.g. red, green or blue. In afurther example, each light extraction features 8 may include afluorescent material (e.g., a phosphor) that that is configured toreceive light in a blue color spectrum and convert it to a differentcolor spectrum (e.g., yellow, green, orange or red) or to a white light(e.g., by mixing the original blue color with the converted color inprescribed proportions). In a further example, light extraction features8 may include a light-scattering material that disperses the incidentlight over a wide angular range. A fluorescent material can be combinedor mixed with a light scattering material. For example, according to oneembodiment, particles of a phosphor material may be mixed together withlight-scattering particles into the ink used to produce light extractionfeatures 8.

According to one embodiment, different individual light extractionfeatures 8 may be formed by different-type inks. For example, lightextraction pattern 101 may have a first series (or sub-pattern) of lightextraction features 8 having only light-scattering particles and asecond series (or sub-pattern) of light extraction features 8 havinglight-scattering particles and color pigments or fluorescent materials.The first and second series (or sub-patterns) of light extractionfeatures 8 may be distributed over surface 11 such the light-scatteringlight extraction features 8 are alternating with the color-pigmented orfluorescent light extraction features 8.

The spatial distribution patterns of the first and second sub-patternsof light extraction features 8 may be different from each other. Forexample, the spatial distribution of the light-scattering lightextraction features 8 may be configured to provide a uniform intensitydistribution of the emission from surface 11 and/or 12 (e.g., using afirst density or density gradient of the respective microdots) and thespatial distribution of the color-pigmented or fluorescent lightextraction features 8 may be configured to provide a uniform colordistribution of the emission from surface 11 and/or 12 (e.g., using asecond density or density gradient of the respective microdots). Thismay be particularly critical for the cases where, for example, the rateof light extraction from light guide 800 in one color is different thanthe rate of light extraction from light guide 800 in another color.

According to one embodiment, at least some light extraction features 8having a color pigment of fluorescent material may be accompanied by oneor more smaller “satellite” light extraction features 8 formed bylight-scattering ink. Two or more different types of light extractionfeatures 8 may also be grouped together to form clusters and configuredto cumulatively emit light (e.g., due to mixing of the individual lightbeams) with a prescribed spectral and/or angular distribution. Onecluster may be configured to emit light with a first spectral and/orangular distribution and another cluster may be configured to emit lightwith a different second spectral and/or angular distribution. Accordingto one embodiment, light extraction features 8 of one type or color maybe formed in surface 11 and light extraction features 8 of a differenttype or color may be formed in opposite surface 12.

According to one embodiment, light extraction pattern 101 may have anon-uniform average areal density (or coverage) of individual lightextraction features 8 at different locations of surface 11 (see, e.g.,FIG. 5 ). The spacing between individual light extraction features 8 maybe varied according to a regular or irregular pattern. Furthermore,light extraction pattern 101 may be segmented into multiple smaller-arealight extraction patterns that are either separated from each other orhave different optical properties and/or distribution densities of lightextraction features. These smaller-area light extraction patterns mayhave distinct boundaries. They can also be separated from each other byspacing or separation areas that are generally free from lightextraction features 8 or have much lower density of light extractionfeatures 8 (e.g., at least by a factor of 2 or more). According to oneembodiment, the spacing or separation areas may be distributed over thearea of surface 11 according to an ordered geometrical pattern, such asfor example, an array of parallel bands.

FIG. 5 illustrates an embodiment of system 900 in which lightextraction/light emitting pattern 101 is segmented into multiple lightextraction/light emitting sub-patterns, as indicated by areas 55. Thosesub-patterns (patterned areas 55) are separated from each other and fromedges of light guiding sheet 10 by separation areas 54 that aregenerally free from light extraction features 8. According to an aspect,patterned areas may be alternating with separation areas 54. The term“separated” should be construed broadly and may include cases whenpatterned areas 55 are discrete objects (areas) completely surrounded bynon-patterned separation areas 54 and cases when two or more patternedareas 55 are connected to each other (e.g., represent portions of asingle, larger patterned area having a complex outline, such as scriptlettering, for example) but have a non-patterned separation area 54between them (e.g., along a straight line that may be drawn between thepatterned areas).

Separation areas 54 (or spacing areas) may be configured in the form ofa perpendicular grid of narrow bands that extend all the way betweenopposing edge surfaces 13 and 14 and 15 and 16. More specifically, thebands representing separation areas 54 are arranged into two parallelarrays that intersect with each other at a right angle. The parallelbands of the first array extend perpendicular to light input edges 13and 14 and the parallel bands of the second array extend parallel tolight input edges 13 and 14. Alternatively, the bands representingseparation areas 54 may be configured to extend diagonally throughsurface 11. Each band or strip representing an individual separationarea 54 may have a width W₅₄ that is less than the length or width ofpatterned light extraction areas 55.

According to some embodiments, it may be preferred that width W₅₄ ofeach separation area 54 is at least several times greater than prevalentspacing distances SPD between light extraction features 8 in areasimmediately adjacent to the respective separation area 54. According tosome exemplary embodiments, width W₅₄ can be at least 3 times, 5 times,10 times, 20 times, 50 times, and 100 times greater than an average SPDcharacterizing the distances between light extraction features 8 in anadjacent patterned area 55. According to some embodiments, it may bepreferred that the overall size of each area 54 (e.g. along its longestdimension) is at least several times greater than a prevalent or averagespacing distances SPD between light extraction features 8 within sucharea, e.g., at least by 3, 5, 10, 20, 50, 100, or 1000 times.

According to some embodiments, the cumulative area of patterned areas 55is less than the area of sheet 10 (or either one of its surfaces 11 and12) by at least 2 times, at least 2.5 times, at least 3 times, at least3.5 times, at least 4 times, at least 5 times, at least 6 times, atleast 8 times, or at least 10 times. According to some embodiments, thecumulative area of patterned areas 55 is less than the cumulative areaof separation areas by at least 2 times, at least 2.5 times, at least 3times, at least 3.5 times, at least 4 times, at least 5 times, at least6 times, at least 8 times, or at least 10 times.

The operation of light guide illumination system 900 of FIG. 5 isillustrated by the example of an exemplary light ray 134. Ray 134 isemitted by one of the LEDs 2, propagated through light guide 800 (sheet10) and extracted using individual light extraction feature 8 of lightextraction pattern 101 such that ray 134 further propagates towards aprescribed direction (e.g., towards a viewer or an object to beilluminated).

According to one embodiment, separation areas 54 may be configured tosuppress or otherwise significantly limit the rate of light extractionin those areas such that the guided light can only be extracted from oneof areas 55. This may be useful, for example, in order to create avisually distinct appearance of light guide 800, particularly whenilluminated by LEDs 2. In a further non-limiting example, separationareas 54 may be configured to provide at least some visual transparencyof light guide 800 even when it is illuminated, regardless of thedensity and/or light-blocking operation of patterned areas 55.

According to one embodiment, wide-area light guide illumination system900 may include a layer having opaque members or materials, andseparation areas 54 may be located in areas of sheet 10 which arecovered by the opaque members or materials. For instance, light guide800 may be associated with a grid or reflective or light absolvingmembers. Such a grid may be exemplified by an egg-crate lightingdiffuser or a grid of parabolic louvers and the opaque members may berepresented by the louvers or the walls of the egg-crate diffuserstructure. Since extracting and emitting light in the areas where theopaque grid members are located could result in a loss of efficiency(e.g., due to light absorption or reflection by the grid members)suppressing the light emission in those areas by providing separationareas 54 may be important for enhancing the overall efficiency of thewide-area light guide illumination system 900. Suitable exemplaryarrangements of waveguide-based illumination systems including segmentedlight extraction features and opaque members and illustrating theprinciples of formation of light extraction features in spaces betweensuch opaque members may be found in U.S. patent application Ser. No.16/679,147 (the '147 Application), the disclosure of which isincorporated herein by reference in its entirety.

According to one embodiment, wide-area light guide illumination system900 may include an opaque sheet-form mask approximately coextensive withsurface 11 and having openings approximating the locations and shapes ofpatterned light extraction areas 55, such that separation areas 54 aredisposed below the opaque portions of the mask and patterned lightextraction areas 55 are disposed below the openings in the mask.According to some embodiments, spacing areas 54 may also be used forattaching other optical elements to light guide 800, such as, forexample, light coupling elements disclosed in the '846 Patent and '666Publications or light guiding elements disclosed in the '423Publication.

According to one embodiment, the arrangements and optical properties ofindividual light extraction features 8 may generally be the same orsimilar for each area 55. According to one embodiment, the arrangementsand/or optical properties of individual light extraction features 8 maybe the different for different areas 55. For example, the areal density,spatial distribution, color properties, sizes and/or shapes of lightextraction features 8 may be made variable from one area 55 to another.More specifically, patterned light extraction areas 55 disposed atgreater distances from a light input edge may generally have greaterareal density of light extraction features 8 compared to patterned lightextraction areas 55 disposed at smaller distances from the light inputedge. Furthermore, the distribution density of light extraction features8 within individual light extraction areas 55 may be increased with adistance from the light input edge.

According to one embodiment, one or more areas 55 may include a layer ofphotoluminescent or phosphor material. Such photoluminescent or phosphormaterial can be configured to absorb light in a first wavelength andre-emit light in a second wavelength which is different than the firstwavelength. According to one embodiment, it may be preferred that thesecond wavelength is greater than the first wavelength. By way ofexample, such material can be configured to absorb at least a portion ofblue light emitted by some types of LEDs and re-emit the energy of suchblue light in another color or in the form of perceptibly white light.

Areas 55 may further incorporate color filters, inks, dyes or otherdevices or substances that change the color of the extracted light. Itmay also incorporate polarizing elements, fluorescent elements, lightscattering or diffusing elements and the like, which may be provided asseparate layers covering areas 55 or incorporated into the bulk materialof light extraction features 8.

Patterned light extraction/light emitting areas 55 may include one ormore shapes cut from a sheet of fluorescent material that convertsshorter wavelength of light in the UV or visible spectrum into longerwavelengths in the visible range. Such sheet-form shapes may belaminated onto surface 11 or surface 12 of sheet 10 (or otherwisepositioned in a close proximity and in front of one of such surfaces),covering respective areas 55, and configured to scatter light withfluorescent effect when illuminated with a light source. According toone embodiment, one or more such shapes can be printed on an opticallytransmissive (e.g., transparent or translucent) substrate, which can belaid on top of surface 11 or surface 12, and positioned in registrationwith areas 55.

By way of example and not limitation, the fluorescent material may beconfigured to convert 350 nm-400 nm UV light from a “black light” intovisible wavelengths (e.g., 500 nm-600 nm). In another non-limitingexample, the fluorescent material may be configured to convert 450nm-495 nm visible (blue) light into visible wavelengths of longerwavelengths (e.g. cyan, magenta, yellow, orange, red, and/or green).Exemplary wavelength ranges of the converted light may include 490-520nm, 500-530 nm, 560-590 nm, 520-560 nm, and 635-700 nm. According to oneembodiment, areas 55 may include a first fluorescent material (e.g.,yellow phosphor) having a first band gap and a second fluorescentmaterial (e.g., red phosphor) having a second bandgap which is differentthan the first bandgap. These different-bandgap materials can be mixedtogether within individual light extraction features 8 or formed asseparate layers on top of areas 55. Alternatively, different-bandgapmaterials can be distributed between different light extraction features8.

According to one embodiment, separation areas 54 may include one or moreshapes cut from a sheet of fluorescent material that converts shorterwavelength of light in the UV or visible spectrum into longerwavelengths in the visible range, e.g., as described above in referenceto patterned light extraction/light emitting areas 55. Such shapes maycover only portions of the separation areas 54 or cover substantiallyentire separation areas 54. According to one embodiment, one or moresuch shapes can be laminated onto surface 11 or surface 12 of sheet 10(or otherwise positioned in a close proximity and in front of one of thesurfaces). According to one embodiment, one or more such shapes can beprinted on an optically transmissive (e.g., transparent or translucent)substrate, which can be laid on top of surface 11 or surface 12, andpositioned in registration with separation areas 54.

Different color-changing materials, such as color filters, inks, dyes orfluorescent particles may be included into patterned lightextraction/light emitting areas 55 to provide different emission colors,e.g., as defined by the CMYK or RGB color spaces. According to oneembodiment, these color changing materials may be mixed together toproduce the desired color effect of the emission. According to oneembodiment, these color changing materials may be distributed amongdifferent light extraction features 8 to achieve a similar color/visualeffect.

In various implementations, color-changing materials may be used inconjunction with other optical materials that do not perceptible changecolors (e.g., light-scattering, reflective or clear inks). According toone embodiment, color-changing materials may be mixed together with suchnon-color-changing materials in different proportions. For example, acolor pigment or fluorescent material (e.g., phosphor) can be mixed withclear and light scattering materials (e.g., UV inks) and individuallight extraction features 8 may be printed using the resulting ink.According to one embodiment, color-changing and non-color-changingmaterials may be provided as separate elements. For example, one or morelight extraction/light emitting area 55 may include a firsttwo-dimensional pattern of printed microdots of color-filtering orcolor-converting ink (which may include pigments or fluorescentparticles suspended in a clear binder and may optionally include lightscattering particles, such as submicron-sized TiO₂ crystals/particles)and a second two-dimensional pattern of printed microdots oflight-scattering ink which is substantially free from the color-changingpigments or fluorescent particles.

The first and second two-dimensional patterns may overlap with eachother, for example, such that at least some color-filtering orcolor-converting light extraction features 8 are alternating with lightextraction features 8 that are substantially free from thecolor-changing pigments or fluorescent particles. The printed microdotsof the second two-dimensional pattern may be configured to extract lightwithout color filtering or conversion and emit light predominantly in aspectral range or spectrum that approximates the spectral range orspectrum of the emission of LEDs 2 (e.g., blue-color light). At the sametime, the printed microdots of the first two-dimensional pattern may beconfigured to emit at least a portion of light in a spectral range thatis different from that of LEDs 2 (e.g., cyan, magenta, yellow, greenand/or red). The printed microdots of the first two-dimensional patternmay be configured to emit light in one spectral range, two distinct oroverlapping spectral ranges, three distinct or overlapping spectralranges, or distinct or overlapping spectral ranges or more than threedistinct or overlapping spectral ranges. The printed microdots of thefirst two-dimensional pattern may be configured to emit at least somelight in a spectral range or spectrum that approximates the spectralrange or spectrum of LEDs 2.

According to one embodiment, both the first and second two-dimensionalpatterns may have a variable density of respective light extractionfeatures 8 across the area of sheet 10 (e.g., with the spacing betweenindividual light extraction features 8 decreasing with a distance fromthe light input edges of sheet 10). According to one embodiment, therate of change in the density of light extraction features 8 within thefirst two-dimensional pattern may be different than the rate of changein the density of light extraction features 8 within the secondtwo-dimensional pattern such that a ratio between the number ofcolor-filtering/converting light extraction features 8 andnon-color-filtering/converting light extraction features 8 per unit areais different for different locations of sheet 10.

According to an aspect of the embodiment of FIG. 5 , surface 11 haslight extracting areas (areas 55) that are alternating with spacing orseparation areas (areas 54) in a repeating pattern. According to oneembodiment, width W₅₄ can be made constant along the entire length ofeach individual separation area 54. According to one embodiment, widthW₅₄ can be made variable along the length of the individual separationarea 54. According to one embodiment, width W₅₄ may also differ from oneseparation area 54 to another.

According to one embodiment, each light extraction feature 8 may beformed by a relatively small dot (a microdot) of a highly diffuselyreflective, light scattering material deposited to surface 11. Themicrodots may be distributed over surface 11 according to an ordered orrandom two-dimensional pattern. Suitable materials for light extractionfeatures 8 may include white inks or paints having a nominal reflectanceof at least 80% in the visual spectrum, preferably having at least 85%nominal reflectance, even more preferably at least 90% nominalreflectance, and still even more preferably at least 95% nominalreflectance. Light scattering dots may be formed by white inks that areradiation-curable (in particular, UV-curable), aqueous (water-based) orsolvent-based. When LEDs 2 are configured to emit light in a particularwavelength range, the ink material should preferably have a nominalreflectance greater than 85%, 90% or 95% in that wavelength range.

The nominal reflectance of a material is directed to mean a totalreflectance (e.g., a percentage of light striking a surface which isreflected off such surface) of the material when the material isprovided at a sufficient “nominal” thickness to form a substantiallyopaque layer (e.g., having the opacity of above 90%). If a material isprovided at a less than the nominal thickness (e. g, when the opacity ofthe respective layer is below 90%), the reflectance may generally belower than the nominal reflectance. For example, the material used toproduce the microdots forming individual light extraction features 8 mayhave a nominal reflectance of 85-95%, but the thickness of thereflective material in the microdot may be significantly less than thenominal thickness, such that the actual measurable reflectance of themicrodots may be in the range from 35% to 65%.

According to one embodiment, individual light extraction features 8 mayhave a volume between 1 picoliter and 10 picoliters. According to oneembodiment, individual light extraction features 8 may have a volumebetween 1 picoliter and 20 picoliters. According to one embodiment,individual light extraction features 8 may have a volume between 10picoliters and 60 picoliters. According to one embodiment, each or atleast some of individual light extraction features 8 may have a volumeof about 1 picoliter. According to one embodiment, individual lightextraction features 8 have a volume of about 3 picoliters. According toone embodiment, individual light extraction features 8 have a volume ofabout 4 picoliters. According to one embodiment, individual lightextraction features 8 have a volume of about 5 picoliters. According toone embodiment, individual light extraction features 8 have a volume ofabout 10 picoliters.

The microdots forming light extraction features 8 may be printed onsurface 11 using a flatbed or roll-to-roll material deposition printer,a UV printer, an ink-jet printer, a sublimation printer, or a screenprinter, for example. According to one embodiment, the white ink mayinclude nanoparticles of titanium dioxide, strontium sulfide, zincsulphide, zink oxide, or other type of white, high-reflectance powdersuspended in a liquid resin or suspension which viscosity is suitablefor the selected type of surface deposition technique (e.g., UVprinting). The nanoparticles may be formed by any type of ahigh-refractive-index material (preferably having n>1.6 and preferablyhaving n>2) and may be configured to scatter light primarily usingdiffraction, at least in one preselected wavelength range. Thehigh-refractive-index material should preferably be opticallytransmissive or even transparent at least at the thicknessescorresponding to the size of light-scattering particles. According toone embodiment, light extraction features 8 may include materials withspecific color-filtering properties (e.g., pigmented inks or fluorescentinks) and can change the color of light.

According to one embodiment, the light-scattering microdots may beformed by a UV-curable ink that includes nanoparticles of opticallytransmissive, high-refractive-index material suspended in a translucentor, more preferably, highly transparent polymerizable binder materialthat has a significantly lower refractive index than that of thenanoparticles. According to one embodiment, the binder material has arefractive index between 1.5 and 1.6. Examples of the transparentpolymerizable binder material binder materials include various acrylatesand their derivatives (e.g., epoxy acrylates, polyurethane acrylates andpolyester acrylates) obtained by reacting an acrylate with a suitableepoxide, urethane or polyester resins. In one embodiment, the bindermaterial may also include a polyester resin or polyurethane resin mixedwith a UV-polymerizable reagent. According to one embodiment, the bindermaterial may include acryl acid ester (e.g., 40-60% by weight) and1,6-Hexanediol diacrylate (e.g., 20-30% by weight).

According to one embodiment, at the time of printing (or otherwisedeposition to a surface of light guide 800), the uncured ink shouldpreferably have a viscosity in the range of 10 to 150 centipoise (cP).If the ink has a higher viscosity at room temperature (25° C.), it maybe heated before surface deposition to bring the viscosity down to theprescribed range. According to one embodiment, the viscosity of theuncured ink at room temperature is between 10 cP and 30 cP. According toone embodiment, the viscosity of the uncured ink at room temperature isbetween 5 cP and 15 cP. According to one embodiment, the viscosity ofthe uncured ink at room temperature is from 5 cP to 25 cP.

According to one embodiment, light extraction features 8 may havephosphorescent or fluorescent properties. For example, the resin orsuspension used to print light extraction features 8 on surface 11 mayinclude a fluorescent material or phosphor that converts a shorterwavelength of light in the ultraviolet (UV) or visible spectrum into oneor more longer wavelengths in the visible range. Such phosphorescentmaterial can be configured to absorb light in a first wavelength andre-emit light in a second wavelength which is different than the firstwavelength. According to one embodiment, it is preferred that the secondwavelength is greater than the first wavelength. By way of example, thematerial may be configured to absorb at least a portion of blue lightemitted by some types of LEDs and re-emit the energy of such blue lightin the form of perceptibly white light.

Light extraction features 8 may be distributed over the designatedarea(s), e.g., patterned light extraction areas 55, according to anordered or random pattern. According to one embodiment, such pattern maybe formed by a two dimensional array of rows and columns. In oneimplementation, every other row or every other column may be shiftedrelatively to the adjacent rows or columns so as to form a staggeredarray or rows or columns. According to one embodiment, the positions ofindividual light extraction features 8 may be randomized within anotherwise ordered pattern. For example, the position of individual lightextraction feature 8 may be slightly offset from the “ideal” position ina two-dimensional ordered pattern (such as an ordered array of rows andcolumns). In one implementation, the amount of the offset can be greaterthan one tenth of the longest or shortest dimension of light extractionfeatures 8 but less than the respective dimension. In oneimplementation, the amount of the offset may be made greater than onetenth of the longest or shortest dimension of light extraction features8 and less than one half of the spacing between adjacent lightextraction features 8. Properly selecting the offset may be particularlycritical in view of reducing the visibility of the light extractionpattern or its visual artefacts (such as moiré effect and the like).

According to one embodiment, light extraction features 8 may bedistributed according to a high-density pattern and have a cumulativearea that approximates the exposed area of surface 11. According to oneembodiment, substantially the entire exposed area of surface 11 may becoated by a continuous layer of a semi-opaque light diffusing material,such as non-absorbing white ink or bulk scattering particles suspendedin a polymeric material, for example. According to one embodiment, oneor more areas 55 may be substantially covered with a continuous layer ofa semi-opaque light diffusing material. According to an aspect, thefully coated area 55 may represent individual broad-area lightextraction feature 8.

According to one embodiment, light extraction features 8 may be formedby a light scattering or light diffusing film that is attached tosurface 11 in the respective areas (e.g., sections 55). Such film shouldpreferably have a hemispherical reflectance of at least 85%, morepreferably at least 90%, and still more preferably at least 95%.

LED chips or dies employed in LEDs 2 may be configured to emit a bluelight. Light extraction features 8 may be configured to change the lightemission spectrum upon interaction with blue light propagating in lightguide 800. For example, a YAG phosphor may be employed in lightextraction features 8 to convert such blue light to a white light (e.g.,by converting a portion of the blue light into longer wavelengths). Thephosphor material may be mixed with silicone or other encapsulationmaterials. Light extraction features 8 may be deposited directly tosurface 11 in a liquid form, for example, by printing, spraying,dispensing, coating or other suitable liquid material depositionmethods.

According to one embodiment, light extraction features 8 are formed bylight-deflecting or light-diffusing surface microstructures formed in oron surface 11. The microstructures may include ordered or random surfacerelief features formed, for example, by means of etching, embossing,laser ablation, sanding, micromachining, micro-replication and any othermethod suitable for producing the desired surface texture or relief.

According to some embodiments, light extraction features 8 may be formedby micro-cavities formed in surface 11. Each micro-cavity may have theshape of a lens, a prism, a blind hole, or can be simply a microscopicdiscontinuity in surface 11 allowing some light to escape from lightguide 800 in the respective location. The size of individual lightextraction features may range from submicron sizes to severalmillimeters or more. According to one embodiment, the size of individuallight extraction features 8 is between 1 micrometer and 25 micrometers.According to one embodiment, the size of individual light extractionfeatures 8 is between 20 micrometers and 100 micrometers. According toone embodiment, the size of individual light extraction features 8 isbetween 40 micrometers and 100 micrometers. According to one embodiment,the size of individual light extraction features 8 is between 40micrometers and 200 micrometers. According to one embodiment, the sizeof individual light extraction features 8 is between 100 micrometers and1 millimeter. According to one embodiment, the size of individual lightextraction features 8 may be made sufficiently small to be virtuallyinvisible to a naked eye, and the spacing between individual featuresmay be made sufficiently large (e.g., two or more times larger than thesize of individual light extraction features 8) so that light guide 800can have a substantially transparent appearance at least whennon-illuminated and when viewed from a normal viewing direction.

According to one embodiment, light extraction features 8 may be formedin a separate film or thin-sheet material which can be then applied tosurface 11 with a good optical contact and preferably with refractiveindex matching. For example, the points of optical contact betweensurface 11 and the film or thin-sheet material may be configured tofrustrate TIR at those points of contact such that light can escape fromwaveguide 800 and enter the film or thin-sheet material, which, in turn.May be configured to further direct and scatter light. According to oneembodiment, light extraction features 8 may be formed in surface 12.According to one embodiment, light extraction features 8 may be formedin both surfaces 11 and 12, for example, to enhance the light extractionrate without allowing individual light extraction features 8 to overlapor become too close to each other (e.g., to prevent light guide 800 tobecome completely opaque in appearance).

According to some embodiments, referring to FIG. 5 , certaincharacteristics of light extraction features 8 or their two-dimensionalpattern in one patterned light extraction area 55 may be different fromthose of another (e.g., adjacent) area 55. For example, the geometricpatterns, relative areas occupied by light extraction features 8, thespacing between adjacent light extraction features 8, the size, shape,thickness, reflectance, absorption, color or fluorescent properties ofvarious light extraction features 8 can be different in different partsof surface 11.

The properties of light extraction features 8 may also vary graduallyacross surfaces 11 and/or 12. According to one embodiment, lightextraction features 8 of surface 11 may be formed by one type of lightdeflecting elements (e.g., by inkjet-printed microdots) while lightextraction features 8 of surface 12 may be formed by a different type oflight deflecting elements (e.g., by surface microstructures formed bymicroimprinting or hot embossing).

Light extraction features 8 are configured to progressively extractlight propagating in sheet 10 and result in a substantially uniformlight emission from either one or both surfaces 11 and 12. In order toachieve a uniform emission, the two-dimensional pattern of lightextraction features 8 can have a variable spatial density in differentareas. According to one embodiment, the density should increase with thedistance from LEDs 2. The density gradient may be selected based on thesize and thickness of sheet 10 and may be determined from opticalraytracing or actual experiments with different-density patterns.

According to one embodiment, separation areas 54 may include lightextraction features 8 having a much lower areal density compared toadjacent patterned light extraction areas 55. According to differentembodiments, the areal density of light extraction features 8 in one ormore separation areas 54 may be less than the areal density of lightextraction features 8 in one or more patterned light extraction areas 55by at least 10%, at least 20%, at least 30%, at least 50%, at least 2times, at least 3 times, at least 5 times, or at least 10 times. Theareal density of light extraction features may also vary within each ofthe areas 54 and 55 (preferably increasing with a distance from nearestLEDs 2).

According to some embodiments, various types of surface structures, suchas light extraction features 8, may be formed in separation areas 54 forpurposes other than (or in addition to) light extraction. For example,surface structures of a predetermined height may be formed in separationareas 54 to prevent unwanted optical contact of surfaces 11 and 12(e.g., by maintaining a minimum spacing) with other optical layers(e.g., optical films), which may be disposed on the surfaces, thuspreventing uncontrolled light escape from waveguide 800. Morespecifically, the height of the structures may be selected to preventevanescent-wave coupling between waveguide 800 and an external opticalsubstrate (e.g., a diffuser or protective plastic sheet). It may beappreciated by those skilled in optics, evanescent-wave coupling (oroptical cross-talk) between two adjacent substrates depends on therefractive index of the substrates and a distance between thesubstrates. For light guide 800 formed from PMMA (acrylic) material, itmay be preferred that the height of the structures is greater than 0.5λ,greater than 0.8λ, greater than λ, greater than 2λ, greater than 3λ, orgreater than 5λ, where X is a wavelength of light emitted by LEDs 2 andpropagating in light guide 800. More generally, it may be preferred thatthe height of the structures is greater than 0.6 μm, greater than 1 μmor greater than 2 μm.

A uniformity U of luminance of a broad-area surface of sheet 10 (e.g.,surface 11 or surface 12) may be defined by the following relationship:U=1−(L_(PEAK)−L_(AVG))/L_(AVG), where L_(PEAK) is a peak luminance andL_(AVG) is an average luminance characterizing the surface. The peakluminance may be measured using spot measurements at different locationsof the respective broad-area surface using a spot luminance meter. Thesampling area for spot measurements may be defined by a circular areacharacterized by a radius that is much smaller than the X and Ydimensions of light guide 800. A preferred size of the sampling area mayalso be defined by the characteristics of the measurement tool, theoverall size of the panel or the intended application (for example, theanticipated viewing distance). In other words, the spot measurementsshould preferably have sufficient granularity to measure surfaceluminance variations across the light-emitting surface of light guide800. According to one embodiment, the sampling area may be greater than100 times the area of individual light extraction features 8 and lessthan 1/10^(th) of a length or width dimension of sheet 10. According toone embodiment, the sampling area may be greater than an area occupiedby a group or cluster of light extraction features 8 including at least10 individual light extraction features 8.

According to one embodiment, luminance uniformity U of light guide 800(as measured at either one of surfaces 11 and 12) is at least 70%, morepreferably at least 80%, even more preferably at least 85%, and yet evenmore preferably at least 90%. According to one embodiment, a differencebetween an average luminance of different patterned light extractionareas 55 is less than 30%, more preferably is less than 25%, even morepreferably is less than 20%, even more preferably is less than 15%, andstill even more preferably is less than 10%.

According to one embodiment, light guide 800 may be formed by twodistinct light guiding layers which represent different opticallytransmissive sheets. According to different implementations, lightextraction features 8 may be formed in one or both of the sheets (lightguiding layers). According to one embodiment, light guide 800 may beformed by sheet 10 which is folded (e.g., using heat bending) at amidpoint and has light extraction features 8 formed in both flaps of thefolded sheet 10.

It should be understood that light sources illuminating the light inputedges (such as opposing edge surfaces 13 and 14) are not limited tolight emitting diodes (LEDs) and may include any continuous or discretelight sources of any known type, including but not limited to:fluorescent lamps, incandescent lamps, cold-cathode or compactfluorescent lamps, halogen, mercury-vapor, sodium-vapor, metal halide,electroluminescent lamps or sources, field emission devices, lasers,etc. Each individual light source may have a linear configuration andinclude a single linear light-emitting element (e.g., a filament LED orhighly elongated LED package) or a relatively small number of linearlight-emitting elements. Each light source may also have two or morecompact light emitting elements incorporated into a linear array. Whenthe light source includes multiple light emitting elements, each of thelight emitting elements may have a compact shape or an extendedtwo-dimensional or one-dimensional (elongated) shape.

According to one embodiment, at least one of the light sources opticallycoupled to sheet 10 includes a laser source emitting a highly collimatedbeam of light. Suitable examples of a highly collimated beam includelight beams having a full width at half-maximum (FWHM) divergence angleof less than 30°, less than 25° degrees, less than 20° degrees, lessthan 15° degrees, less than 10° degrees, and less than 5° degrees.According to one embodiment, at least one of the light sources opticallycoupled to sheet 10 includes an LED or laser source emitting amoderately collimated beam of light having a FWHM divergence angle ofless than 60°, less than 50° or less than 45°. According to someembodiments, the above-referenced FWHM divergence angles may be definedand measured in a plane that is perpendicular to a prevalent plane ofsheet 10 (e.g., in the YZ plane when light is input through edge surface13).

LEDs 2 may be further associated with integrated or external optics suchas collimating or light-redistributing lenses, mirrors, lens arrays,mirror arrays, light diffusers, waveguides, or optical fibers. Whenmultiple light emitting elements are employed, each of the lightemitting elements may be provided with individual optics. Alternatively,a single linear optic may be provided for the entire array to collimatelight or otherwise shape the emitted beam in a plane which isperpendicular to the longitudinal axis of the array.

Light guide illumination system 900 may include a cover of housingconfigured to encase LEDs 2 and optionally portions of sheet 10 adjacentto the light input edge(s). Such housing may have different functionsincluding but not limited to structural, protective (from dust,moisture, elements, impact, etc.) and/or aesthetic. According to oneembodiment, the housing can be made from a heat-conductive material(e.g., aluminum) which extends over a portion ow sheet 10. LEDs 2 may beaffixed to the heat-conductive housing using fasteners orheat-conductive adhesive.

The overall dimensions of wide-area light guide illumination system 900,the size and shape of sheet 10, the types of LEDs 2 as well as theirnumber, spacing and nominal power may be selected based on the targetapplication. According to one embodiment, light guide illuminationsystem 900 may be configured as a flat-panel lighting luminaire. Thelighting panel may be formed into curved shapes and curved-shapeluminaires, e.g., as described in any of the '972, '097, '841, and '538Patents. According to another embodiment, it may be configured as abacklight of an LCD display. According to a yet another embodiment, itmay be configured as an illuminated sign, artwork or image print or as abacklight for such devices. According to a yet another embodiment, itmay be configured as an illumination system (e.g., backlight orplanar-panel light) for a planar panel photobioreactor.

Patterned light extraction areas 55 may have different rectangularshapes and arranged on surface 11 according to a different patterncompared to those of FIG. 5 . Furthermore, light extraction pattern 101may include a mix of different shapes and sizes of areas 54 and 55. Thisis further illustrated in FIG. 6 , which schematically depicts variousexemplary geometrical configurations of areas 55 within light extractionpattern 101 and also schematically depicts other exemplary dimensionsand arrangements of LEDs 2.

According to one embodiment, each area 55 may have a hexagonal shape. Anexemplary arrangement of such hexagonal areas 55 within light extractionpattern 101 is schematically shown in FIG. 7 . It should be understoodthat the depicted pattern of hexagonal (or other-shaped) patterned lightextraction areas 55 may extend continuously over broad-area surface 11both longitudinally and laterally (along the X and Y directions) tocover sufficiently large areas. The sizes and density of lightextraction features 8 within each hexagonal area 55 may be selected suchthat an average luminance of different parts of pattern 101 is about thesame or similar, e.g., within 20% of the average luminance produced bythe entire patterned surface of sheet 10.

According to one embodiment, LEDs 2 may include individually digitallyaddressable RGB or RGBW (e.g., red-green-blue-white) LEDs. Suchindividually digitally addressable LEDs 2 may be selectively turned onand off or dimmed to illuminate select areas of stepped light guideillumination system 900 in different brightness and/or color.

According to one embodiment, individual light extraction features 8 orgroups of light extraction features 8 can be made individuallycontrollable and dynamically changing their color or light extractionproperties in response to an external factor or signal (e.g., suppliedvoltage, magnetic field, electric field, static electricity,illumination by an external source of light, mechanical or opticalcontact with an external object, etc.). Such individually controllablelight extraction features 8 or groups may be selectively turned on andoff or dimmed to illuminate select areas of light guide illuminationsystem 900 in different brightness and/or color.

According to one embodiment, individual areas 55 may be configured andindividually controlled as individual “pixels” within a largeilluminated LED display. Such LED display may incorporate hundreds andthousands of areas 55. For example, each pixel including a singleindividually controllable area 55 may have a size from 0.5 mm to 5 mm orfrom 1 mm to 10 mm.

FIG. 8 schematically illustrates an exemplary light extraction pattern102 including light extraction features 8 formed in patterned lightextraction areas 55 and light extraction features 9 formed in separationareas 54 that are alternating with areas 55 along the Y axis. The arealdensity of light extraction features 9 within areas 54 of pattern 102 ismuch lower than the areal density of light extraction features 8 withinareas 55 of the pattern. The spatial distributions of light extractionfeatures 8 within each area 55 has a variable density. At least someareas 55 have a positive gradient of the areal density towards a centralaxis. Similarly, the spatial distributions of light extraction features9 within each area 54 has a variable density with a positive gradient ofthe areal density towards a central axis. The areal density of lightextraction features 8 and/or 9 may be expressed, for example, in termsof the number of respective light extraction features per unit area.According to an aspect, light extraction pattern 102 includesalternating bands having different areal densities of light extractionfeatures 8 and 9. Furthermore, the areal density is variable within eachband. According to one embodiment, the areal density within each ofareas 55 and 54 may have a positive gradient along a light propagationpath (e.g. a light path from LEDs 2 coupled to a nearest light inputedge). Furthermore, an average areal density may increase from one area55 to another area 55 and/or from one area 54 to another area 54 alongthe light propagation path.

A relative surface area of light extraction features 8 and/or 9 at anyparticular location of surface 11 and/or surface 12 may be defined as asum of the individual areas of light extraction features 8 and/or 9within a selected sampling area divided by the total area of thesampling area. For example, a relative surface area of 0.5 correspond toone-half of the respective sampling area being cumulatively occupied bythe light extraction features (50% areal coverage). A relative surfacearea equal to one means that the light extraction features occupy 100%of the sampling area, with no spaces between adjacent light extractionfeatures. Depending on the size and shape of individual light extractionfeatures, the spatial density and relative surface area may be bound byvarious predefined relationships.

According to one embodiment, light extraction patterns of features 8and/or 9 are characterized by spacing distances SPD which progressivelydecrease with a distance from LEDs 2 at least within some samplingareas. The sampling areas may be taken at any locations of patternedlight extraction areas 55 or separation areas 54. According to oneembodiment, spacing distances SPD progressively decrease with a distancefrom LEDs 2 within at least some sampling areas. According to oneembodiment, the light extraction patterns of light extraction features 8and/or 9 are characterized by spacing distances SPD which progressivelydecrease with a distance from LEDs 2 within at least a first samplingarea and progressively increase with a distance from LEDs 2 within atleast a second sampling area that is different from the first samplingarea.

According to one embodiment, spacing distances SPD progressivelydecrease with a distance from LEDs 2 within at least some patternedlight extraction areas 55. According to one embodiment, a first averagespacing distance SPD characterizing a first patterned light extractionarea 55 located at a first distance from a light input edge (e.g., edgesurface 13) is greater than a second average spacing distance SPDcharacterizing a second patterned light extraction area 55 located at asecond distance from a light input edge, which is greater than the firstdistance. According to different embodiments, a difference between thefirst and second average spacing distances SPD spacing can be at least10%, at least 20%, at least 30%, at least 40%, at least 50%, or at least100%.

FIG. 8 illustrates varying the spatial density of light extractionfeatures 8 and 9 (and, hence, varying the relative area occupied bylight extraction features 8 and 9) by varying the spacing betweenindividual light extraction features. However, it should be understoodthan the relative area may also be varied by varying the size ofindividual features 8 and/or 9, even at a constant spacing. For example,increasing the area of each light extraction feature 8 or 9 by two timeswithin a particular sampling area will increase the relative area ofsuch features within the sampling area by two times.

According to one embodiment, LEDs 2 may be side-emitting LEDs. In someimplementations, the side-emitting LEDs may be attached directly tosurface 11 or 12 of sheet 10 (e.g. glued using a two-sided adhesivetransfer tape). Examples of side emitting LEDs that may be suitable forLEDs 2 include but are not limited to Micro SIDELED product seriescommercially available from OSRAM (e.g., LW Y87C, CUW Y3SH.B1 and LWY1SG models of white LEDs or LB Y8SG model of blue LEDs) or modelsNS2W364G and NS2W266G of white side-emitting LEDs manufactured byNichia.

Light can be injected/coupled into light guide/waveguide 800 not onlythrough outer perimeter edges (e.g., edge surfaces 13, 14, 15, and 16),but also through any of surfaces 11 and 12 or inner edges which may beformed between the outer perimeter edges. According to one embodiment,sheet 10 may include a number of cutouts 77 (FIG. 6 ). At least some oftop-emitting or side-emitting LEDs 2 may be inserted into respectivecutouts 77 and optically coupled to one or more edges of the cutouts toilluminate waveguide 800 from the inside through those edges. Accordingto one embodiment, side-emitting LEDs 2 may be provided on a rigidheat-spreading PCB substrate. According to one embodiment, side-emittingLEDs 2 may be provided on a flexible heat-spreading PCB substrate. ThePCB substrate may be bonded to light guiding sheet 10. The PCB substratemay also be bonded to a support substrate disposed behind light guidingsheet 10. The support substrate may be coextensive with light guidingsheet 10. Suitable examples of such support substrates include but arenot limited to a reflector sheet, transparent or translucent glass orplastic sheet, or a structural members such as aluminum extrusion.

Cutouts 77 may take the form of parallel channels or rectangularcavities aligned along an edge of sheet 10 (e.g. edge 16), see FIG. 6 ).At least some of the channels or rectangular cavities may also belocated between patterned light extraction areas 55 (e.g., in separationareas 54), see, e.g., FIGS. 1-6 of the '423 Publication. Cutouts 77 mayalso take the form of discrete round openings in light guiding sheet 10(waveguide 800) appropriately dimensioned to accommodate the size ofindividual side-emitting LED 2 (e.g., as disclosed in U.S. Pat. No.9,256,007 in reference to FIGS. 35, 36A and 37B). For LEDs 2 havingelongated rectangular shapes, cutouts 77 may have elongated rectangularshapes/outlines and may have slightly larger dimensions than therespective dimensions of the bodies of LEDs 2 (e.g., to accommodate theinsertion of LEDs 2 into the cutouts). Cutouts 77 may be separated fromeach other and from edges of light guiding sheet 10 by portions ofbroad-area surfaces 11 and/or 12. According to an aspect, cutouts 77 maybe exemplified by through holes formed in light guiding sheet 10 inselected locations. The through holes may be of a rectangular shape(with sharp or rounded corners), round shape, elongated oval shapes orother shapes (e.g., free-forms) and may likewise be dimensioned toaccommodate the respective dimensions and shapes of LEDs 2. Cutouts 77may also be configured to assist in positioning LEDs 2 relatively tosheet 10 and facilitate optical coupling (e.g., by preventing lateraland transversal shifting of sheet 10 and LEDs 2 relatively to eachother).

Referring further to FIG. 6 , sheet 10 may include light couplingelements 67 which may also be referred to as light couplers or lightinjectors. According to some embodiments, light coupling elements 67 maybe exemplified by sheets 20 or optical couplers 88 of the '423Publication, optical elements 6 of the '846 Patent, and/or lightcoupling elements 2 of the '666 Publication. Light coupling elements 67(along with respective LEDs 2) may be provided outside light extractionpattern 101 or inside light extraction pattern 101. Light couplingelements 67 (along with respective LEDs 2) may be provided at outerperimeter edges of sheet 10, at its inner edges (e.g., adjacent tocutouts 77) or at any locations of surfaces 11 and 12, includingpatterned light extraction areas 55 and separation areas 54.

It is noted that the embodiments of wide-area light guide illuminationsystem 900 described herein may also be adapted to using many differenttypes and form factors of side-emitting or top-emitting LEDs and mayfurther be adapted to many different types, shapes (e.g., square, roundor rectangular), configurations and architectures of LEDs, alsoincluding packageless LEDs, for example. Furthermore, non-LED lightsources can be used in place of LEDs 2, such as, for example, lasers,fluorescent lamps, incandescent lamps, gas-discharge lamps, and OLEDs.LEDs 2 may incorporate LED arrays or arrays of LED die assembled withina single package. Suitable examples of such LEDs as well as relatedmethods of LED coupling to light guides (waveguides) are disclosed, forexample, in the '666 Publication. Additional exemplary embodiments ofLEDs and light coupling structures that can be used to input light intolight guide 800 are disclosed in the '423 Publication. “Example 1” inthe '423 Publication further discloses an exemplary configuration oflight extraction patterns for obtaining a substantially uniform lightemission from the entire light emitting area of a planar-type lightguide, which can be applied for patterning light guide 800 of thisinvention, according to at least some embodiments.

FIG. 9 schematically depicts a portion of light guiding sheet 10 andindividual light extraction feature 8 exemplified by a fully-cured,solidified drop (microdot) of a UV-curable ink deposited to surface 11.It is noted that the illustrative example of FIG. 9 may also be appliedto configuring light extraction features 9 discussed above and mayfurther be applied to embodiments in which light extraction features 8and/or 9 are formed in surface 12.

Referring FIG. 9 , the UV-curable ink forming respective lightextraction feature 8 includes a highly transparent, UV-reactive binder37 and a suspension of high-refractive-index, light-scattering particles33. A suitable example of light-scattering particles 33 having a highrefractive index includes submicron-sized particles of titanium dioxide.Particles 33 are about evenly distributed throughout the volume ofUV-reactive binder 37 and provide volumetric bulk scattering propertiesfor light extraction feature 8. Some particles 33 may form agglomerates35 in which a number of particles 33 may be disposed in contact witheach other or at a very close distance to one another. Such distance canbe much smaller than the average distance between individual particles33 in binder 37. Agglomerates 35 may form two- or three-dimensionalstructures that have sizes from a fraction of the micrometer to severalmicrometers. Light-scattering particles 33 may be randomly varied insize within a single light extraction feature 8. Furthermore, anyindividual light extraction feature 8 may include a number of randomlyformed agglomerates 35 that represent localized regions of increaseddensity of light-scattering particles compared to surrounding areas.According to one embodiment, at least some of light scattering particles33 include one or more fluorescent materials. Fluorescent materials mayalso be mixed with non-fluorescent (e.g., color filtering or lightscattering) materials in various proportions, e.g., 10%: 90%, 50%: 50%,or 90% 10%.

According to one embodiment, light extraction feature 8 of FIG. 9 mayhave an irregular elongated shape having a length L₈ and a maximumheight H₈. Maximum height H₈ may also be referred to as a maximumthickness of light extraction feature 8. A transverse width of theelongated light extraction feature 8 may be 1.2 times, 1.5 times, 2times, 2.5 times, 3 times, 5 times or 10 times less than length L₈. Anaspect ratio (length to width ratio) may vary randomly from one lightextraction feature 8 to another. According to one embodiment, elongatedlight extraction features 8 may be arranged in groups having generallythe same orientation of a longitudinal axis. In one embodiment, theorientations of elongated light extraction features 8 may be randomwithin a predefined angular range. The angular range may be 10 degrees,30 degrees, 45 degrees, 60 degrees, 90 degrees, 180 degrees, and 360degrees (e.g., a completely random orientation).

A surface 36 of light extraction feature 8 that is exposed to air has amicrostructured surface including random microstructures 34.Microstructures 34 produce a non-negligible surface roughness that ismuch greater than the roughness of surface 11 and that contributes torefractive or diffractive light scattering or dispersion produced bylight extraction feature 8. The surface roughness may be characterizedaccording to an American National Standard ASME B46.1-2009.

The surface roughness may be selected to maximize the diffractive lightscattering operation of light extraction feature 8. According todifferent embodiments, an RMS surface roughness parameter R_(q) ofsurface 36 is greater than 30 nanometers, greater than 40 nanometers,greater than 60 nanometers, approximately equal to or greater than 100nanometers, and approximately equal to or greater than 200 nanometers.At the same time, it may be preferred that the RMS surface roughnessparameter R_(q) of surface 36 measured along the same sampling length isone of the following: less than 1 micrometer, less than 0.5 micrometers,and less than or equal to 0.3 micrometers.

According to different embodiments, the parameter R_(q) can be measuredalong one of the following sampling lengths: 10 micrometers to 20micrometers, 20 micrometers and 40 micrometers, 20 micrometers to 100micrometers, and 5 micrometers to 200 micrometers. When measuring theroughness of surface 36, the profile waviness that characterizes theshape of surface 36 should normally be subtracted before evaluatingR_(q). The definition of surface waviness is illustrated in FIG. 10which shows a surface profile (as it may be measured by a surfaceprofilometer, for example) and a centerline characterizing the overallshape of the object and which should be subtracted from the measuredprofile to determine parameter R_(q).

The surface waviness may be customarily subtracted by high-passfiltering with a cut-off wavelength λ_(f) (see e.g., ASME B46.1-2009).The cut-off wavelength λ_(f) should be at least 5 to 10 times less thanthe sampling length. On the other hand, the cut-off wavelength λ_(f)should be at least several micrometers, for example, 2 to 5 micrometersor 5 to 20 micrometers. The upper limits for cut-off wavelength λ_(f)may also be defined by the size of light extraction feature 8. Forexample, cut-off wavelength λ_(f) may be set to at most the length L₈ orone-half of the length La.

According to one embodiment, the parameter R_(q) of surface 11 should beless than 25 nanometers, more preferably less than 20 nanometers, evenmore preferably less than 15 nanometers and still even more preferablyless than or equal to 10 nanometers. The measurements of parameter R_(q)of surface 11 may be performed in a vicinity of light extraction feature8. The measurements should preferably utilize the same or similarsampling length as that used for measuring R_(q) of surface 36.According to one embodiment, the parameter R_(q) of surface 11 should beperformed along a direction that is perpendicular to a light input edge(e.g., perpendicular to edge surface 13).

According to some embodiments, the size of light extraction feature 8may range from 10 micrometers to 200 micrometers, from 30 to 150micrometers, from 30 to 80 micrometers or from 100 to 500 micrometers inthe longest dimension. According to different embodiments, the volume ofat least some of printed microdots forming individual light extractionfeature 8 may range from 1,000 cubic micrometers to 10,000 cubicmicrometers, from 1,000 cubic micrometers to 100,000 cubic micrometers,from 10,000 cubic micrometers to 100,000 cubic micrometers, from 20,000cubic micrometers to 80,000 cubic micrometers, and from 30,000 cubicmicrometers to 60,000 cubic micrometers. According to one embodiment,the volume of at least some of individual printed microdots is about4,000 cubic micrometers. According to one embodiment, the volume of atleast some of individual printed microdots is between 15,000 cubicmicrometers and 20,000 cubic micrometers. According to one embodiment,the volume of each light extraction feature 8 formed by one or moreprinted microdots is between 2,000 cubic micrometers and 6,000 cubicmicrometers.

The size of individual random surface microstructures 34 may range fromseveral nanometers to several micrometers. According to a preferredembodiment, microstructures 34 may have sizes of less than 0.5 microns.According to one embodiment, at least some microstructures 34 may havetop portions that can be approximated by spherical shapes having aradius of curvature between 50 nanometers to 200 nanometers.

According to one embodiment, each light extraction feature 8 may includea generally opaque material (e.g., white pigment) but at such a lowthickness that the light extraction feature 8 is semi-opaque,non-absorbing and transmits at least a portion of light impinging on it.The term “non-absorbing”, in reference to opaque or semi-opaque layersof various materials used for making light extraction features 8 or 9,such as white pigment inks, for example, is directed to mean that therespective layer(s) do not perceptibly absorb light. For instance, anexemplary non-absorbing layer may be formed by an optically clear resinloaded with submicron TiO₂ particles at 10-20% concentration (byweight). The non-absorbing layer may have a relatively low thicknessbetween about 1 micrometer and 10 micrometers. Due to the low thicknessand the presence of submicron TiO₂ particles, the layer can beconfigured to partially reflect light and partially transmit lightwithout perceptible absorption. Accordingly, the sum of a totalreflected light energy E_(R) and a total transmitted light energy E_(T)can be equal (preferably within 1%-5% error) to a total light energy E₀incident onto the non-absorbing layer (E₀=E_(R)+E_(T)). According todifferent embodiments, the absorption within the semi-opaque material ata thickness equivalent to the average thickness of light extractionfeatures 8 and/or 9 is less than 5%, more preferably less than 3%, evenmore preferably less than 2% and still even more preferably less than1%.

As employed in the present specification and claims, the term “opacity”refers to the extent to which a surface, an object or a layer of amaterial impedes the transmission of light through it. For example, alayer or surface that completely prevents light passage is consideredcompletely opaque and having a 100% opacity. In contrast, a layer orsurface that transmits essentially all of the incident light isconsidered having a 0% opacity. Accordingly, a layer or surface thattransmits one-half of the incident light is considered having a 50%opacity. Light extraction features 8 and/or 9 may have a relatively highdegree of opacity (e.g., 30%, 50%, 60%, 70%, 80%) and, at the same time,can be substantially non-absorbing (e.g., the sum of the energy of thetransmitted and reflected light can be approximately equal to the energyof the incident light).

For a partially opaque (semi-opaque) surface or layer which is alsoreflective and non-absorbing, the opacity may be defined and measured asa ratio of the reflectance of the surface or layer against a lightabsorbing background surface to its reflectance against a highlyreflective background surface, at least according to some embodiments.The light absorbing background surface should preferably have <5%reflectance and even more preferably <3% reflectance. The highlyreflective backing surface should preferably have a hemisphericalreflectance of at least 89%, more preferably at least 95% and even morepreferably 98-99%. The highly reflective backing surface may bespecularly reflecting, diffusely reflecting or reflect light bothspecularly and diffusely.

In an alternative, according to some embodiments, the opacity may bemeasured using standard techniques, such as those described in ASTMD1746-15 or ASTM D589-97 documents and using a suitable standardizedopacity meter. When it is impossible to directly measure the opacity ofindividual semi-opaque light extraction features 8 or 9 (e.g., due totheir small size), the measurements may be performed indirectly using abroad-area layer of the same semi-opaque material deposited to atransparent substrate with a uniform thickness corresponding to aweighted average thickness of the material in the individual lightextraction features 8 and/or 9. For example, if a weighted averagethickness of the semi-opaque light extraction features is 8 micrometers(as measured, for instance, by a 3D microscope or a profilometer), atransparent substrate may be coated with a uniform 8-micrometer-thicklayer of the same (or very similar) material, and the opacitymeasurements may be performed for that layer in order to characterizethe opacity of the light extraction features 8.

In a further alternative, the opacity may be expressed and measured interms of light attenuation by the material of light extraction features8 and/or 9. More specifically, the opacity may be defined by thefollowing expression: 100%(1−I_(T)/I₀), where I₀ is the intensity oflight incident onto the semi-opaque layer of light extraction featureand I_(T) is the intensity of light that is transmitted through thesemi-opaque layer. According to one embodiment, the opacity and/orreflectivity of the semi-opaque layers may be measured and/or comparedin accordance with one or more other applicable standards, such as, forexample, ISO 2814, ISO 6504, BS 3900-D4, BS 3900-D7, ASTM E97, ASTME1347, ASTM D4214, ASTM D2805, and ASTM D589.

For the purpose of characterizing and configuring light extractionfeatures 8 and/or 9, the opacity may provide a useful measure of thefraction of light impinging onto such light extraction features from theside of light guiding sheet 10 that can be emitted from the other sideof the light extraction features (e.g., away from surface 11 whenfeatures 8 and/or 9 are formed in that surface).

According to one embodiment, referring to FIG. 9 , a desired opacity oflight extraction feature 8 may be provided by scattering visible lightin all directions in a tree-dimensional space using relativelytransparent particles 33 volumetrically distributed within the bulk ofbinder material 37. Preferred mechanisms for scattering light using suchparticles and without perceptible absorption include refraction,diffraction, reflection or a combination thereof.

According to one embodiment, light scattering particles 33 suspended inbinder material 37 may be formed by spherically shaped nanoparticles ofa relatively transparent material having a very high index of refraction(n≥2). Suitable exemplary materials particularly include but are notlimited to inorganic white pigments such as rutile or Anatase titaniumdioxide (n=2.5-2.8), antimony oxide (n=2.1-2.3), Zinc Oxide (n≈2), whitelead (basic lead carbonate, n=1.9-2.1), and lithopone (n=1.8).Alternatively, other inorganic materials or polymers having moderatelyhigh refractive indices (about 1.6 or greater) may also be used,including particularly magnesium silicate, baryte, calcium carbonate,calcium carbonate, polystyrene, or polycarbonate.

The sizes of particles 33 may be selected to maximize light scatteringin a particular range of wavelengths (e.g., visible wavelengths centeredaround 0.5 micrometers). Suitable sizes of particles 33 to maximizediffraction can be calculated, for example, using the principles of Mietheory of light scattering. According to one embodiment, an average sizeof particles 33 is about 200 nanometers. According to one embodiment, anaverage size of particles 33 is about 250 nanometers. According to oneembodiment, an average size of particles 33 is between 100 nanometersand 400 nanometers. According to one embodiment, the size of particles33 is between 150 nanometers and 350 nanometers.

Two or more different sizes (or size distributions) of particles 33 canalso be mixed within the material of the ink used to produce lightextraction features 8. Such mixing may be directed to maximize lightscattering at two or more different wavelength ranges. These differentwavelength ranges may be non-overlapping (e.g., 400-450 nm and 570-590nm) or overlapping (e.g., 400-550 nm and 450-600 nm).

The concentration of particles 33 in binder material 37 may vary in abroad range. According to a preferred embodiment, the concentration ofparticles 33 (pigment loading) is between 5% and 35% by weight or volumein the respective ink material used to produce light extraction features8. According to one embodiment, the concentration is between 30% and 50%by weight. According to one embodiment, the concentration is between 5%and 25% by weight. According to one embodiment, the concentration isbetween 10% and 20% by weight. According to one embodiment, theconcentration is from 1% to 5% by weight. According to one embodiment,the concentration may be less than 1% by weight.

According to some embodiments, light extraction features 8 and/or 9 maybe substantially non-absorbing and may have an opacity of at least 10%,at least 20%, at least 30%, at least 40% or at least 50%. According tosome embodiments, the opacity of at least some individual lightextraction features 8 and/or 9 is less than 90%, approximately equal toor less than 80%, approximately equal to or less than 70%, orapproximately equal to or less than 50%. According to one embodiment,the opacity of individual light extraction features 8 is between 30% and70%. According to one embodiment, the optical transmittance ofnon-absorbing, semi-opaque light extraction features 8 is one of thefollowing: greater than 10%, greater than 20%, greater than 30%, greaterthan 40% and equal to or greater than 50%.

According to one embodiment, the opacity of the material forming lightextraction features 8 (e.g., white or fluorescent UV-curable ink) isbetween 40% and 70% when measured at a layer thickness of 10 micrometersor less. According to alternative embodiments, the opacity is between30% and 50% when measured at a layer thickness of 0.5 micrometers to 5micrometers, 1 micrometer to 2 micrometers, or 1 micrometer to 6micrometers.

A further useful measure of the opacity of light extraction features 8and/or 9 and a measure of the ability of light guiding sheet 10 to emitlight from both sides (e.g., from both opposing surfaces 11 and 12) is aluminance ratio between the respective opposing surfaces. For example,let's consider an embodiments of wide-area illumination system 900 inwhich light extraction features 8 are formed by printing microscopicopaque or semi-opaque white-ink dots on surface 11. In an illustrative,non-limiting example, when light guiding sheet 10 is patterned withlight extraction features 8 and illuminated from one or two edges byLEDs 2, an average measured surface brightness of surface 11 may beabout 4,500 cd/m² while an average measured surface luminance of surface12 may be about 1,500 cd/m², thus giving a 3 to 1 ratio. In other words,light guiding sheet 10 will emit about 75% of light from surface 11 andabout 25% from opposing surface 12. For the purpose of determining theopacity and/or light transmittance of light extraction features 8 and/or9 using this method, the surface luminance should preferably be measuredfrom a perpendicular direction with respect to surfaces 11 and/or 12.

According to one embodiment, the opacity, light transmittance and lightscattering properties of light extraction features 8 are configured suchthat each of surfaces 11 and 12 outputs 30% to 70% of the total lightemitted by sheet 10 through surfaces 11 and 12. In other words, surface11 may be configured to output no less than 30% of the total lightenergy emitted from sheet 10 as a result of light extraction by lightextraction features 8, and, at the same time, surface 12 may also beconfigured to output no less than 30% of the total light energy emittedfrom sheet 10 as a result of such light extraction. According todifferent embodiments, the approximately proportions between lightoutput from surfaces 11 and 12 may be 30%: 70%, 35%: 65%, 40% 60%, 45%:55%, 50%: 50%, 55%: 45%, 60%: 40%, 65%: 35%, 70%: 30%, or any ranges inbetween.

According to some embodiments, the opacity of light extraction features8 may be selected such that the ratios between the measured luminance ofsurfaces 11 and 12 are about 1:1 (about equal surface luminance ofsurfaces 11 and 12), 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1,1.8:1, 1.9:1, 2:1, 3:1, 3.5:1, 4:1, 4.5:1, and 5:1. According to someembodiments, the opacity of light extraction features 8 may be selectedsuch that the ratio between the measured total output from surface 11and surface 12 is about 1:1 (about equal amounts of light are emittedfrom both surfaces), 1.1:1, 1.2:1, 1.3:1, 1.4:1, 1.5:1, 1.6:1, 1.7:1,1.8:1, 1.9:1, 2:1, 3:1, 3.5:1, 4:1, 4.5:1, and 5:1. Such configurationswith two-sided light output may be advantageously selected, for example,for applications that will benefit from direct/indirect illumination.Furthermore, configurations of systems 900 with semi-opaque lightextraction features 8 may be advantageously selected for applicationswhich could benefit from the smallest size and/or thickness of the lightextraction features and which require that the light extraction patternis virtually invisible to a naked eye at relatively close viewingdistances.

It is noted that the use of semi-opaque light extraction features is notlimited to the cases of two-sided illumination. FIG. 9 furtherillustrates the operation of system 900 when semi-opaque, substantiallynon-absorbing light extraction feature 8 is used together with areflective back sheet. Reflective sheet 41 should preferably have aspecular or diffuse hemispherical reflectance of at least 80%, morepreferably 85-90%, and even more preferably 90-98%.

In operation, a light ray 301 exemplifies light that isextracted-decoupled from light guiding sheet 10 and distributed/emittedfrom wide-area illumination system 900 using a multi-stage process. Ray301 initially propagates in light guiding sheet 10 (light guide 800) ina waveguide mode. Subsequently, ray 301 enters semi-opaque lightextraction feature 8 and encounters one of light scattering particles 33which splits the energy of ray 301 into two distinct portionspropagating toward different directions. A first portion of ray 301 isforward-scattered by one of particles 33, forming a ray segment 303. Asecond portion of ray 301 is forward-scattered by particle 33, forming aray segment 302.

Ray segment 302 is further forward-scattered by an adjacent particle 33and is directed towards surface 12 (not shown) at an angle below thecritical TIR angle characterizing surfaces 11 and 12, so it can befinally extracted from sheet 10 and emitted from surface 12. Ray segment303 is further emitted from surface 36 and reflected back towards sheet10 by reflective sheet 41. Ray segment 303 re-enters the body of lightextraction feature 8 and is further propagated back into sheet 10,undergoing additional interactions (and forward scattering) with one ormore light scattering particles 33. Ray segment 303 enters onto sheet 10at a below-TIR angle, allowing for decoupling ray segment 303 from lightguide 800.

According to an aspect, ray segment 302 exemplifies light that isback-scattered by light extraction feature 8 towards surface 11 and thateventually re-enters light guiding sheet 10 and can be emitted fromopposing surface 12 (not shown), contributing to the total usefulemission from system 900. Ray segment 303 exemplifies light that isinitially forward-scattered away from surface 11 and that contributes tothe emission from surface 11. Accordingly, in the absence of reflectivesheet 41, the total light emission from sheet 10 would be distributedbetween surfaces 11 and 12 according to a certain ratio. This ratio canbe determined and controlled, at least in part, by the opacity of lightextraction feature(s) 8. In the illustrated example, ray segment 303 isintercepted by highly reflective sheet 41 and reflected back towardslight guiding sheet 10. Ray segment 303 further passes through thesemi-opaque layer of light extraction feature 8 for the second time,undergoing some additional scattering, so that it can be ultimatelyemitted from opposite surface 12 of light guiding sheet 10, alsocontributing to the emission from that surface.

It is noted that individual light rays being extracted by semi-opaquelight extraction features 8 may undergo multiple back- andforward-scattering deflections within the light-scattering layer, e.g.,as schematically illustrated by various segments of ray 301 in FIG. 9 .Additionally, the extracted light rays may further undergo multiplepassages through the same light extraction feature 8 or even differentlight extraction features 8. The number of interactions of the extractedlight ray with light-scattering particles 33 may depend on the size ofsuch particles, their distribution density within the semi-opaque layer,height H₈, spacing between light extraction features 8, and otherfactors. The thickness, opacity, light transmittance and the size ofparticles 33 may be selected such that the light scattering is maximized(for example, by maximizing light diffraction by particles 33 andproviding for at least double or multiple passage of light through thematerial of light extraction features 8) while light absorption withinthe body of light extraction features 8 is minimized.

According to one embodiment, the light scattering provided bysemi-opaque, semi-transmissive light extraction features 8 with orwithout the aid of reflective sheet 41 is such that light emitted fromlight guiding sheet 10 has a Lambertian or quasi-Lambertian angulardistribution. In other words, the luminous intensity observed fromsurface 12 of light guiding sheet 10 can be approximately proportionalto the cosine of the observation angle (an angle between the observationdirection and a surface normal). According to one embodiment, theangular emission distribution can be approximated by a Lambertian cosinelaw at least in the YZ plane. According to one embodiment, the angularemission distribution can be approximated by a Lambertian cosine law atleast in the XZ plane. According to one embodiment, the angular emissiondistribution can be approximated by a Lambertian cosine law in both XZand YZ planes. According to some embodiments, the ratio between aluminous intensity of surface 12 measured at 45° from the surface normaland a luminous intensity of surface 12 measured along a normal directionis from 0.5 to 0.95, from 0.7 to 0.9, from 0.75 to 0.9, or from 0.8 to0.9. By way of example and not limitation, the luminance intensitymeasured from a perpendicular direction to surface 12 may be 500-600 cdand the luminance intensity measured from a 45° angle may be 400-500 cd.

According to some embodiments, light guide illumination system 900 ofFIG. 9 may be used without a reflector (e.g., reflective sheet 41) andcan still be configured for providing a Lambertian or quasi-Lambertianangular distribution of light emission, e.g., from surface 11, surface12 or both surfaces 11 and 12. According to some embodiments, lightextraction features 8 may be configured to provide a “bat-wing” angulardistribution of light emission from either one or both surfaces 11 and12, e.g., by appropriately configuring the opacity, transmittance andlight scattering properties of the cured ink layer which forms the lightextraction feature.

The average thickness of the semi-opaque layer of each micro-printedlight extraction feature 8 may be selected to provide a prescribed ratiobetween forward-scattering and back-scattering. In order to maintain aminimum prescribed level of light transmittance for light extractionfeatures 8, height H₈ (maximum thickness) or the average thickness ofthe respective semi-opaque layer may be limited to certain values. Insome embodiments, height He or the average thickness may be equal to orless than 60 micrometers, equal to or less than 50 micrometers, equal toor less than 30 micrometers, equal to or less than 20 micrometers, equalto or less than 15 micrometers, equal to or less than 10 micrometers,less than 8 micrometers, equal to or less than 6 micrometers, equal toor less than 5, equal to or less than 3 micrometers, equal to or lessthan 2, between 1 and 2 micrometers, between 1 and 1.5 micrometers,about 1 micrometer, or between 0.5 and 1 micrometers. At the same time,according to at least some embodiments, the characteristic size (e.g.,dimension L₈ or diameter d) of features 8 may range from 30 micrometersto 150-200 micrometers and more preferably from 30 micrometers to 140micrometers. According to different embodiments, a prevalent size ordiameter of light extraction features 8 is about 50 micrometers, 60micrometers, 70 micrometers, 80 micrometers, 90 micrometers, 100micrometers, or 100-150 micrometers. According to one embodiment, theprevalent size or diameter of light extraction features 8 is between 50micrometers and 80 micrometers. According to one embodiment, the maximumthickness of the semi-opaque layer of light extraction feature 8 may begreater or equal to 1 micrometer and less than or equal to 10micrometers. According to one embodiment, the maximum thickness may havevalues from 1 micrometer to 8 micrometers, from 1 micrometer to 6, from2 micrometer to 6, or from 2 micrometer to 4 micrometers.

According to at least some embodiments, the ratios between L₈ and He maybe as low as 3 or as high as 100 or so. According to some embodiments,the ratio between L₈ and He may be between 5 and 100, between 10 and200, between 10 and 100, between 20 and 100, between 40 and 100, andbetween 40 and 80.

According to one embodiment, highly reflective sheet 41 is configured toreflect primarily by a specular reflection (such that the angles ofreflection are about equal to the angles of incidence). According to oneembodiment, highly reflective sheet 41 is configured to reflectprimarily in a diffuse regime and thus provide additional scattering forthe extracted light compared to scattering/deflecting light using lightextraction features 8 only.

The opacity of individual light extraction features 8 and the density ofsuch light extraction features across surface 11 and/or 12 may beconfigured to control the opacity of light guiding sheet 10 in differentareas. In an extreme exemplary case, the density of light extractionfeatures 8 can be made very high (e.g., from 80% to 100%, such thatthere is practically no spacing between individual light extractionfeatures 8 (e.g., separation distances SED are about zero or less thanzero) and that light guiding sheet 10 or its portions are substantiallyopaque. In a further example, by selecting an even higher packingdensity (with separation distances SED being significantly less thanzero but greater than −d/2) the opacity of light guiding sheet 10 may bemade similar to that of individual light extraction features 8.

According to one embodiment, light extraction features 8 may be formedby stretchable ink. In different implementations, the fully cured inkmaterial should allow for its reversible stretching without cracking inthe elastic or plastic mode by at least 10%, 30%, 50%, 10%, 150%, or200% elongation.

Example 1

FIG. 11 shows an exemplary measured dependence of the opacity of lightguiding sheet 10 on spacing between light extraction features 8. In theillustrated example, light guiding sheet 10 was formed by an opticallyclear acrylic sheet having a 0.75-mm thickness. Light extractionfeatures 8 were formed by depositing microdrops of UV-curable white inkon a surface of the acrylic sheet using a commercial flatbed UV printerwith instant curing of the respective micro-droplets to a solid formusing UV light.

The individual printed light extraction features 8 had sizes around120-130 micrometers along the longest dimensions and an average totalthickness of about 6-8 micrometers. In each sample pattern, lightextraction features 8 were arranged in a two-dimensional array having afixed spacing (pitch) in the X and Y dimensions (fixed spacing distanceSPD for each sample). Multiple samples of patterned light guiding sheets10 were produced using different spacing distances SPD between lightextraction features 8.

The patterns having microdrop spacing SPD below 100 micrometerscompletely covered the surface with the ink (reaching a 100% fillfactor) due to overlapping of adjacent microdrops, which corresponded tothe case of near-zero or slightly negative separation distances SED. Ata microdrop pitch of 84 micrometers (SPD=0.084 mm), the printing processproduced a continuous layer of white ink with a measured averagethickness of 7-8 micrometers (substantially overlapping microdrops withseparation distances SED being significantly less than zero). At evenlower SPD values and even greater overlap of the printed microdrops, thethickness of the resulting layer was measured at about 30 micrometerswith approximately uniform coverage.

Referring further to FIG. 11 , relatively sparsely populated, discretelight extraction features 8 (e.g., spaced by distances SPD of 0.3 mm to0.6 mm) produced fairly low levels of opacity for light guiding sheet10/light guide 800. On the other hand, densely populated lightextraction features 8 (e.g., spaced by distances SPD of 0.1 mm andbelow) produced moderate to relatively high levels of opacity for thelight guiding sheet.

At a pitch (or spacing distance SPD) of about 42 micrometers, theopacity of sheet 10 reached a maximum at about 66%. Accordingly, at thisopacity level, light guiding sheet diffusely reflected about two thirdsof the incident light and diffusely transmitted around one third of theincident light. The absorption within the layer of white ink was foundto be less than 3-5% (less than or comparable to the measurementerrors).

According to an aspect, each light guiding sheet 10 patterned withseparation distances SED being about zero or less also produced asemi-opaque light diffuser having a relatively thin (2-30 μm) lightscattering layer which was virtually lossless (e.g., non-absorbing).Accordingly, light guiding sheet(s) 10 may also be configured forefficiently diffusing light using minimal amounts of raw materials.Moreover, considering that the opacity, reflectance and transmittance ofsheet 10 may be controlled by the thickness of the ink layer and thatthe angular directionality of the transmitted light may be controlled bythe size, refractive index of forward-scattering particles mixed intothe UV ink, the operation and optical parameters of the resultinglighting diffuser may be varied in a broad range. According to differentembodiments, light guiding sheet 10 may be configured as a non-absorbinglight diffuser having a transmittance of greater than 50%, greater than60%, greater than 70%, or greater than 80% and may further be configuredto scatter a parallel beam of light over an angle of at least 30°, atleast 40°, at least 45°, at least 60°, at least 90°, at least 120°, atleast 140°, or at least 160° (in a transmissive operation).

The graph of FIG. 11 can be approximated by a polynomial function andinterpolated or extrapolated to calculate areal pattern coverage toachieve a prescribed level of opacity. In view of such results, it isnoted that, according to at least some embodiments, the opacity ofsemi-opaque light extraction features 8 may also be approximatelydetermined by measuring the opacity of light guiding sheet 8,particularly in the areas of relatively high density of light extractionfeatures 8 and extrapolating the results to the 100% coverage of thesurface with the respective semi-opaque layer, at the appropriate layerthickness.

End of Example 1

The term “surface coverage” may be defined as the ratio between acumulative area of light extraction features 8 within a sampling regionand the total area of the sampling region. The area of the samplingregion should normally be at least 100 times greater than the average ortypical area of individual light extraction features 8. According to oneembodiment, light guiding sheet includes at least one region having asurface coverage of at least one of the following: 10% or more, 20% ormore, 30% or more, 40% or more, 50% or more, 60% or more, 70% or more,80% or more, and 90% or more. According to some embodiments, spacingareas between light extraction features 8 cumulatively occupy less than50% of the total light guide area within a sampling region, less than30%, less than 20%, or less than 10%. According to one embodiment, lightguiding sheet 10 includes at least one region having the surfacecoverage of at least one of the following: less than 10%, less than 7%,less than 5%, and less than 2%.

Light guiding sheet 10 may be configured such that a first lightextraction pattern of light extraction features 8 is formed in surface11 and a second light extraction pattern of light extraction features 9is formed in surface 12. The second pattern of light extraction features9 may cover the same area of sheet 10 as the first pattern of lightextraction features 8. The first and second patterns may superimpose onone another in terms of X and Y coordinates of the respective individuallight extraction features 8 and 9. The second pattern may also be arotated and/or translated copy of the first pattern. According to oneembodiment, the first and second patterns may have different pitch ordifferent surface distributions of the respective light extractionfeatures 8 and 9, for example, for the purpose of reducing the chance ofthe so-called moiré effect when a light emitting surface of system 900is observed by a viewer.

The shapes, sizes, distribution densities and orientations of lightextraction features 8 and/or 9 may vary in a broad range. According toone embodiment, light extraction features 8 and/or 9 may have randomshapes, sizes and/or orientations across respective surfaces 11 and/or12. This may also be useful, for example, for reducing the conspicuityof the patterns and providing a perceptibly uniform light output fromillumination system 900.

FIG. 12 schematically shows various exemplary shapes, densities andorientations of various light extraction features that can be formed inor on surfaces 11 and 12 of light guiding sheet 10. Light extractionfeatures 701, 702, 705, and 706 may exemplify various-shape microdots ofwhite UV-curable ink printed on surface 11. More particularly, lightextraction features 701 may exemplify regular or quasi-regular shapes ofthe microdots which may be have some small-scale shape irregularities.Light extraction features 702 exemplify printed microdots havingirregular or highly irregular shapes. Light extraction feature 706exemplifies regular or quasi-regular shapes of microdots that have asomewhat fussy outline. Light extraction features 705 exemplify printedmicrodots that have much smaller sizes compared to larger lightextraction features 701, 702, and 706. The smaller-sized lightextraction features 705 are provided in spaces between the large-sizedlight extraction features and can have a generally differentdistribution pattern than the larger light extraction features. In someconfigurations of sheet 10, the areas occupied by the patterns of lightextraction features 701, 702, 705 and 706 may partially or entirelyoverlap in any combination. In some configurations, the areas may beadjacent to ether or spaced apart from one another.

According to one embodiment, light extraction features 705 may be formedby microdots of a first type of UV ink (e.g., a first pigment or a firstphosphor) and configured to emit light in a first spectral range orcolor (e.g., blue color) when illuminated by LEDs 2. Light extractionfeatures 701 may be formed by microdots of a second type of UV ink(e.g., including a second phosphor or second pigment) configured to emitlight in a second spectral range or color (e.g., red color) whenilluminated by LEDs 2. Light extraction features 702 may be formed bymicrodots of a third type of UV ink (e.g., including a third phosphor orthird pigment) configured to emit light in a third spectral range orcolor (e.g., green color) when illuminated by LEDs 2, and lightextraction features 706 are formed by microdots of a fourth type of UVink (e.g., light-scattering particles) configured to emit light in athird spectral range or color when illuminated by LEDs 2 (e.g. thespectral range or color of the emission of LEDs 2). According to oneembodiment, at least some of light extraction features 701, 702, 705 or706 may be formed by optically clear UV ink.

Light extraction features formed by different-type UV ink materials maybe distributed in different relative proportions (e.g., by number,size/density or area) over different areas of surface 11 to createvarious visual effects (e.g., emission in different colors, brightnesslevels, uniformity levels, patterns, etc.) in those areas. Each type ofUV ink may be printed according to a surface distribution pattern whichis different than the distribution patterns of the other types of UV inkand which density generally increases with a distance from a light inputedge (or edges, if multiple) of sheet 10. For example, light extractionfeatures 705 may be distributed over surface 11 according to a firstdistribution pattern having a first density gradient (e.g., an increasein the surface density of the light extraction features with a distancefrom a light input edge), light extraction features 701 may bedistributed over surface 11 according to a second distribution patternhaving a second density gradient which is different than the firstgradient, light extraction features 702 may be distributed over surface11 according to a third distribution pattern having a third densitygradient which is different than the first gradient, and so on.According to one embodiment, the density gradient of the spatialdistribution of light extraction features 705 along the optical path inlight guiding sheet 10 may be greater than the density gradient of thespatial distribution of other types of light extraction features (e.g.,features 701 and 702). This embodiment may be particularly advantageousfor an exemplary case when at least some of LEDs 2 are configured toemit a monochromatic light in a first spectral range, light extractionfeatures 705 are formed by light scattering ink, and the other-typelight extraction features (e. g., 701 and 702) are formed by inksincluding phosphors or other types of fluorescent materials responsiveto the first spectral range (e.g., being configured to absorb light inthe first spectral range and convert the absorbed light into one or moredifferent spectral ranges). For example, light extraction features 705may be configured to separately control the light extraction rate andemission in the first spectral range relatively to the light extractionrate and emission in those different spectral ranges.

According to one embodiment, individual patterns of light extractionfeatures formed by different types or compositions of UV ink may beconfigured to homogenize light output from surface 11 by means of mixingthe light energy and/or color. According to one embodiment, theindividual patterns are configured to produce a homogenous surfaceemission such that the brightness/luminance and/or color of each portionof surface 11 is about the same (e. g., within 5%, 10%, 15%, or 20%).Considering that different types of UV ink will generally deplete lightwithin light guiding sheet 10 at a different rate relatively to eachother, homogenizing the light output from surface 11 and/or 12 may benon-trivial and require more than just routine experimentation. Forexample, it may be appreciated that light extraction features having thesame composition but located at different distances from a light inputedge of light guiding sheet 10 patterned using several different colorsof ink may emit light with different spectral distributions.Furthermore, interactions of one type of light extraction features withlight which has already been deflected, scattered, filtered orcolor-converted by other-type light extraction features nay produceunexpected effects and should be accounted for. Accordingly, a method ofhomogenizing the emission from surface may include adjusting the lightemission in one color, intensity and/or angular distribution at onelocation of surface 11 by adjusting the density and optical/geometricalproperties of light extraction features at that location while alsoadjusting the density and optical/geometrical properties of more thanone type/composition of light extraction features at that one and otherlocations of surface 11.

According to one embodiment, light extraction features 701, 702, and 706may have volumes between 30,000 and 100,000 cubic micrometers while eachof light extraction features 705 can have a volume below 10,000 cubicmicrometers, below 5,000 cubic micrometers, between 1,000 cubicmicrometers and 5,000 cubic micrometers, or less than 1,000 cubicmicrometers.

Referring to FIG. 10 , a fully cured micro-drop of UV-curable ink may becharacterized by a contact angle which is defined, by an analogy fromthe wettability characterization of liquid drops on a solid substrate,as the angle between a curved surface of the micro-drop and thesubstrate surface (e.g., surface 11) where the respective surfaces meet.When the micro-drop has a rough, microstructured surface, the surfaceroughness may be subtracted from the surface profile before measuringthe contact angle. For example, a suitable waviness profile may be usedas a representation of the shape of the micro-drop and the contact anglemay be measured using such waviness profile.

According to one embodiment, at least some of light extraction features8 and/or 9 are formed by UV-cured micro-drops having a contact angle ofabout 10 degrees. According to one embodiment, the contact angle isabout 5 degrees. According to one embodiment, the contact angle is about15 degrees. According to one embodiment, the contact angle is about 20degrees. According to one embodiment, the contact angle is between 2degrees and 5 degrees. According to one embodiment, the contact angle isgreater or equal to 1 degree and less than or equal to 2 degrees.According to one embodiment, the contact angle is less than 1 degree.According to one embodiment, the contact angle is greater than 0.1degree and less than 1 degree. According to one embodiment, the contactangle is between 5 degrees and 25 degrees. According to one embodiment,the contact angle is between 5 degrees and 20 degrees. According to oneembodiment, the contact angle is between 10 degrees and 20 degrees.According to one embodiment, the contact angle is less than 5 degrees.According to one embodiment, the contact angle is greater than 25degrees.

It is noted that the contact angle of liquid (uncured) micro-drops, maybe about the same but may also be generally different from that of thefully-cured, solidified micro-drops. Furthermore, at least someembodiments may include printed light extraction features that areformed by solvent-based micro-drops in which case the contact angles ofthe liquid and fully-cured micro-drops may differ significantly due tothe evaporation of the solvent during the curing process. According toone embodiment, light extraction features 8 are formed by microdrops ofeither radiation-curable or solvent-based materials which are depositedto the respective surfaces using known methods other than printing. Forexample, such materials may be deposited to surface 11 in the form ofliquid microdrops using tightly controlled pressure spraying, ultrasonicspraying, or a combination thereof.

According to one embodiment, a method of making wide-area illuminationsystem 900 may include a step of generating a two-dimensional pattern ofdiscrete locations (e.g., X, Y coordinates) of light extraction features8 where spacing distances between the discrete locations graduallyincrease or decrease with a distance from an edge of the pattern. Thetwo-dimensional pattern may be generated, for example, usingcomputer-based modeling, such as raytracing. The optical modeling may beconstrained to provide a substantially uniform expected light outputfrom light guide 800.

The method of making wide-area illumination system 900 may furtherinclude a step of converting the pattern of discrete locations to acomputer readable raster bitmap having a two-dimensional array ofpixels. Suitable examples of computer readable raster bitmaps that canbe used to store coordinates of light extraction features 8 include, butare not limited to bitmap image file format (e.g., the BMP file format),device independent bitmap (e.g., DIB), tagged image file format (TIFF),portable document format (PDF), JPEG file format, portable networkgraphics (PNG) file format, graphics interchange format (GIF), and thelike. According to one embodiment, it may be preferred that the rasterbitmap is stored in a bitonal (e.g., black-and-white) form. For example,in a black-and-white bitmap, pixels corresponding to the locations lightextraction features 8 may be white and pixels corresponding to spacingareas can be black, or vice versa.

The method of making wide-area illumination system 900 may furtherinclude steps of providing a UV printing machine (a UV printer),providing a light guide substrate (e.g., highly transparent acrylicsheet), loading UV-curable inks of a preselected color (e.g. whiteUV-curable inks) into the UV printing machine, loading the bitmapcontaining information on the locations of light extraction features 8into a software that is used to control the UV printing machine,printing the bitmap on the light guide substrate using the UV printingmachine, and curing the droplets deposited onto the surface of the lightguide substrate (e.g., using UV light from UV LED sources). The curingprocess can be performed simultaneously with the printing process(instant curing) or as a separate post-printing step (delayed curing).

The method of making wide-area illumination system 900 may furtherinclude various software and/or printer configuration steps, such as,for example, specifying the print resolution, target print areadimensions, volume of individual droplets (e.g., based on the targetsize of the printed dots/microdots), and ink curing regimes (e.g., theintensity of UV lamp or time delay for curing). According to oneembodiment, printing can be performed at a resolution of 600 dots perinch (DPI). According to one embodiment, printing can be performed at aresolution of 1200 DPI, 1800 DPI, 2400 DPI or a higher DPI. According toone embodiment, printing can be performed at a resolution of 300 DPI orlower. The DPI resolution of the bitmap associated with the print may beselected to match that of the printing resolution. The size of printeddots may be specified directly, e.g., via appropriate software settings,or indirectly, e.g., by selecting appropriate printing regimes thatresult in a prescribed size of the individual microdots. The surface tobe patterned (e.g., surface 11 or 12) may be conditioned for an enhancedreceptivity of ink and improved wettability. For example, it may bepre-washed in de-ionized water with added surfactants, cleaned in anultrasonic batch or treated using corona discharge or atmosphericplasma.

The formation of individual microdrops within the printer may ordinarilybe performed using a drop-on-demand print head that electricallyactuates a piezoelectric crystal to produce ink drops of a prescribedsize in response to voltage pulses. According to one embodiment, it maybe preferred that each actuation of the piezoelectric crystal results indepositing a single drop of ink to each prescribed location of thesurface being patterned (e.g., the location corresponding to anindividual “white” or “black” pixel in the raster bitmap), such thatindividual light extraction features 8 may be produced by curing thatdrop to a solid state.

According to one embodiment, the method of making wide-area illuminationsystem 900 includes a continuous or at least intermittent recirculationof the ink during the printing process. Including the recirculation stepor process can be especially important for white ink compositions thatinclude heavy particles suspended in a much lighter liquid material(e.g., light-scattering nanoparticles of TiO₂ suspended in a clearUV-reactive acrylate resin/binder material). Without the recirculation,the heavy particles may cause various issues such as, for example, inksedimentation in the fluid paths, clogging the jetting nozzles, andcreating non-uniformities of volumetric particle loading in the ink.This, in turn, may impact ink discharge and severely affect or eveneventually prevent the formation of single-droplet microdots (individuallight extraction features 8) of the prescribed size and volume. Thecontinuous recirculation may be performed continuously or intermittentlyby agitating the dispersion or suspension of the heavy particles withina closed-path independent fluid circuit using a recirculation pump. Theclosed-path fluid recirculation circuit may be located anywhere alongthe ink supply line, e. g, between a reservoir containing the ink andthe printing head. According to one embodiment, the closed-path fluidrecirculation circuit may also be built into the printhead.

The print head may incorporate multiple individual print heads bundledtogether and forming a wide-area multi-element print head which canpattern a relatively large area in a single pass. The area may be fromseveral millimeters to several tens or even hundreds of centimeters inwidth. For example, the wide-area multi-element print head may have aspan equal to a width of the area to be patterned. Alternatively, theprint head may have an active printing area which is much smaller thanthe area to be patterned and may be configured to pattern such area inmultiple passes. According to one embodiment, the print head isconfigured for printing in a reciprocal motion in one dimension and thesubstrate to be patterned (e.g., light guiding sheet 10) can beincrementally fed in an orthogonal dimension in regular intervals.

According to one embodiment of the method of making wide-areaillumination system 900, depositing of different microdots within arelative small area (e.g., within a band having a width from severalmillimeters to several centimeters) may be performed using two or moreconsecutive printing passes of the print head over the same area.According to one embodiment, the method of making wide-area illuminationsystem 900 includes setting a prescribed number of individual drops tobe deposited to the same location. This printing regime can beadvantageously selected for building the prescribed thickness of lightextraction features using a finite pre-defined number of relativelysmall microdots. According to one embodiment where individual lightextraction features 8 are formed using multiple microdrops of UV curableink, each microdrop may be cured to a solid form before depositing anext microdrop on top of it or with a light offset (e.g., to formoverlapping microdots). According to one embodiment, at least somemicrodrops (or layers of light extraction feature 8) may be uncured orpartially cured (e.g., by reducing the intensity of the UV lamp ortuning it off) before depositing a next microdrop on top of it or with alight offset. This may be useful, for example, for enhancing theadhesion between the layers forming light extraction feature 8.

According to one embodiment, the method of making wide-area illuminationsystem 900 may further include bending the light guide substrate (lightguide 800) to a curved shape and making it operable for distributinglight while being in a bent or curved state. According to oneembodiment, the light guiding substrate may be sandwiched between aflexible back-sheet reflector and a flexible, film-thickness opticallytransmissive diffuser sheet (e.g., for a single-sided diffuse emission),all of which can be bent and flexed together with the light guidingsubstrate. The back-sheet reflector may specular or diffuse and mayconventionally have a film thickness for enhanced flexibility. Accordingto one embodiment, the back-sheet reflector may be replaced with anotherflexible, film-thickness transmissive diffuser sheet (e.g., for atwo-sided diffuse emission). According to one embodiment, lightextraction features 8 may be located on a concave side of curved lightguide 800. According to one embodiment, light extraction features 8 maybe located on a convex side of curved light guide 800. According to oneembodiment, light extraction features 8 may be located on both theconvex and concave sides of curved light guide 800.

FIG. 13 schematically illustrates an embodiment of wide-areaillumination system 900 in a front light configuration with asubstantially single-sided emission. Referring to FIG. 13 , there isprovided an image print 755 having a full-color viewable surface 757.Image print 755 is separated from light guiding sheet 10 by a spacinglayer 290 which may be filled with ambient air or an opticallytransparent material having a lower refractive index that the materialof light guiding sheet 10. Light extraction features 8 are configuredfor illuminating surface 757 in a reflective mode of operation. Surface11 is facing away from image print 755 and configured as a front viewingsurface of system 900 through which image print 755 can be observed froma distance. Surface 12 is facing towards image print 755 and configuredfor illuminating the image print. According to one embodiment, imageprint may be spaced apart from light guiding sheet 10 by a distance thatis greater than the thickness of light guiding sheet 10 (e.g., at leastby 1.5 times, 2 times, 3 times, 5 times or 10 times).

It is noted that the front light implementation of illumination system900 is not limited to illuminating an image print and may be used forilluminating space in front of surface 12 (e.g., a room in a building)or illuminating any types of objects or surfaces. In differentembodiments, image print 755 may be replaced with a textured surface,graphics, pattern, indicia, logo, sign, letters, background surface(white, monochrome or colored), fabric, conventional printed image,stereoscopic image, photograph, or LCD display. According to oneembodiment, a three-dimensional object or surface may be used in placeof image print 755. According to one embodiment, a layer of fluorescentmaterial may be used in place of or in conjunction with image print 755.

Each light extraction feature 8 of FIG. 13 is formed by an innerreflective layer 777 and an outer opaque layer 778. The inner layer 777is formed by a semi-opaque, highly reflective material which includestransparent binder material 37 and high-refractive-index lightscattering particles 33 distributed throughout the volume of bindermaterial 37. According to one embodiment, inner layer 777 is formed bylight-scattering UV curable ink. Outer layer 778 is formed by a highlyopaque material having a sufficient thickness to block at least asubstantial portion of light that may be escaping from inner layer 777.According to one embodiment, the highly opaque material may include alight absorbing material. The opacity of outer layer 778 is preferablygreater than 75%, more preferably greater than 80%, even more preferablygreater than 85%, even more preferably greater than 90%, even morepreferably greater than 95%, and still even more preferably greater than97%. According to one embodiment, the opacity of outer layer 778 issubstantially 100%.

Suitable materials for light absorbing outer layer 778 may include, forexample, black or dark-colored ink containing a carbon black pigment.Suitable materials for reflective outer layer 778 may include, forexample, UV-curable or solvent-based metallic ink (e.g., silver oraluminum nano- or micro-particle ink), aluminum foil, a relatively thicklayer of material having a high-brightness reflective pigment, shinyglitter (which can be colored or non-colored), and various types ofmetallic particles, flakes or powder (e.g., gold, silver, bronze oraluminum). The outer layer 778 may be further coated with a protectivelayer (e.g. clear UV cured ink or lacquer).

Inner light-reflecting layer 777 is facing towards image print 755 andouter absorptive layer 778 is facing towards a viewer 660. It may beappreciated that, by utilizing the inner layer which is highlyreflective, light scattering and semi-opaque (also preferably beingbright-white in color) and the outer layer which is light-absorbing andhighly opaque, illumination system 900 may be configured to suppress oreven completely eliminate glare associated with a light emission fromlight extraction features 8 towards viewer 660. Accordingly, thisconfiguration may be advantageously used as a front light forilluminating image print 755 with a high contrast and with minimalglare.

For example, it can be shown that, if the opacity of inner layer 777 is70% and the opacity of outer layer 778 is 90%, the combined opacity canbe about 97% (resulting in only 3% of light being emitted towards viewer660). When the reflectance of inner layer 777 is 60% or more, a ratiobetween light energy emitted towards image print 755 and towards viewer600 may be as high as 20. Accordingly, if an average reflectance ofimage print 755 is 50%, a visual contrast between illuminated imageprint 755 and light extraction features may be as high as 10 (the visualcontrast may be defined as a ratio between the luminance of the visiblesurface of image print 755 and the visible surface of light extractionfeatures 8). According to different embodiments, the total (combined)opacity of layered light extraction features 8 is one of the following:90%, 92%, 94%, 96%, 98%, and 99%.

Opaque layer 778 of light absorbing material is conformably coating thesurface of the semi-opaque inner layer 777 such that minimum orvirtually no light is emitted from the respective light extractionfeature 8 directly towards viewer 660, even when illumination system 900is fully lit by LEDs 2 coupled the respective edges of light guidingsheet 10. According to one embodiment, light extraction features 8 areconfigured to be essentially invisible to viewer 660 when illuminationsystem 900 is in the “on” (illuminated) state. This can be achieved, forexample, by making the size of individual features 8 sufficiently smalland by making the opacity of respective outer layers sufficiently high.According to one embodiment, light extraction features 8 are also madeessentially invisible to a viewer 900 when illumination system 900 is inthe “off” state (i.e., when not illuminated by LEDs 2). According to oneembodiment, light extraction feature 9 are also essentially invisible toa viewer 900 when illumination system 900 is either in the “off” or “on”state. According to one embodiment, light extraction features 8 are madeessentially invisible to viewer 660 at a normal viewing distance whenillumination system 900 is a non-illuminated state but visible at thesame distance when system 900 is in an illuminated state. The normalviewing distance may be defined as a distance from which image print isdesigned to be viewed. In a non-limiting example, for illuminationsystem 900 configured as an information display, the normal viewingdistance may be from 50 cm to 10-20 meters.

It may be preferred that opaque layer 778 is formed on top of thethree-dimensional structure of semi-opaque inner layer 777 as aconformable coating. The term “conformable” with respect to a coatingrefers to a layer that generally conforms in shape to the underlyingthree-dimensional surface, layer and/or structure, such as a curvedouter surface of the inner layer 777 of light extraction feature 8 ofFIG. 13 , for example. Outer opaque layer 778 preferably has thedimensions and shape approximating those of the inner semi-opaque layer777 so as to completely cover inner layer 777. According to differentembodiments, outer layer 778 covers at least 80%, at least 90%, or atleast 95% of the surface of inner layer 777. According to oneembodiment, outer layer 778 covers 100% of the surface of inner layer777.

According to one embodiment, outer opaque layer 778 may have slightlylarger dimensions or slightly larger area (e.g., by 5-10%) than those ofinner layer 777, for example, to ensure that no perceptible amount ofstray light can escape from light extraction feature 8 towards theviewer. According to one embodiment, at least some outermost portions ofouter layer 778 of light absorbing material are disposed in contact withsurface 11 such that outer layer 778 completely encapsulates inner layer777. On the other hand, the size of layer 778 should be limited toreduce unwanted extraction and absorption of light propagating in lightguide 800. According to different embodiments, it is preferred that awidth W_(CA) of optical contact area of outer layer 778 with surface 11on either side of inner layer 777 is less than 30%, less than 20% orless than 10% of the diameter of the surface structure formed by innerlayer 777.

The operation of wide-area illumination system 900 in aglare-suppressing front-light configuration is further illustrated inFIG. 13 by the example of a path of a light ray 341. Ray 341 propagatingin light guiding sheet 10 in a waveguide mode enters binder material 37where it optically interacts with one or more light scattering particles33. A light ray segment 343 exemplifies a portion of light ray 341 thatis forward-scattered upon such interaction. A light ray segment 342exemplifies a portion of light ray 341 that is back-scattered (diffuselyreflected back towards sheet 10 and towards image print 755).

Ray segment 342 propagates back to light guiding sheet 10 where itovercomes TIR at surface 12 and illuminates surface 757 of image print755. Surface 757 further reflects and scatters light exemplified by raysegment 342 and directs the reflected light towards viewer 660.Accordingly, viewer 660 can see image print 755 being illuminated withhigh contrast and without glare that could be otherwise caused by straylight emanated from light extraction feature 8 in the absence of opaquelayer 778.

Ray segment 343 propagates further towards outer layer 778. In anembodiment where outer layer 778 is light-absorbing, ray segment 343 maybe substantially absorbed by that layer. In an embodiment where outerlayer 778 is reflective (e.g., formed by reflective ink or metallicfoil), at least a substantial portion of the energy of ray segment 343(e.g., at least 30%, at least 40%, at least 50%, at least 60%, or atleast 70%) may be reflected (optionally with some scattering) by thatlayer and may re-enter light guiding sheet 10 at below-TIR angle (withrespect to a normal to surface 12) and illuminate image print 755.

To prevent or minimize the visibility of individual light extractionfeatures 8 or to enhance the visibility of image print 755 at relativelyshort viewing distances (e.g., 50 cm or less for signage-typeapplications), the light extraction features should preferably besmaller than 150-200 micrometers, more preferably smaller than 100-150micrometers, even more preferably smaller than 100 micrometers, andstill even more preferably smaller than 80 micrometers. In someinstances, however, e.g., when the viewing distances are one meter,several meters or more, the size of light extraction features 8 may beselected to be 300 micrometers or more, 0.5 millimeter or more, 1millimeter or more, and up to several millimeters or more.

The spacing distances SPD between light extraction features 8 shouldpreferably be equal to or less than the combined thickness of lightguiding sheet 10 and spacing layer 290 such that individual light beamsformed by light extraction features 8 could overlap on one another atsurface 757. This may be particularly critical for achieving arelatively uniform illumination of image print 755 According to someembodiment, the combined thickness of light guiding sheet 10 and spacinglayer 290 is greater than the spacing distances SPD between at leastsome adjacent light extraction features 8 by at least 1.5 times, atleast 2 times, at least 2.5 times, at least 3 times, at least 5 times,at 10 times, or at least 20 times.

The opacity of the material forming outer light absorbing layer 778should preferably be significantly greater than the opacity of thematerial forming the inner layer of light extraction features 8.According to one embodiment, outer layer 778 may include a colorpigment. It may also include highly reflective (e.g., metallic)particles in concentrations sufficient to provide enhanced opacity forthe layer. According to one embodiment, opaque outer layer 778 may havereflective properties and provide enhanced opacity due to reflectionrather than absorption. According to one embodiment, opaque outer layer778 may be replaced with a highly reflective layer that likewiseprovides enhanced opacity. In this case, light rays striking therespective opaque layer (e.g., ray segment 343 of FIG. 13 ) may berecycled by reflecting such rays back towards light guiding sheet 10 andimage print 755. This configuration may be advantageously selected toenhance system efficiency and the apparent brightness of illuminatedimage print 755 compared to the case of employing outer layer 778 in alight-absorbing configuration.

Light extraction features 8 and/or 9 may include other layers havingvarious useful functions. According to one embodiment, an opticallytransmissive (or transparent) layer 779 may be provided betweenparticle-loaded binder material 37 and surface 11, as furtherillustrated in FIG. 13 . Optically transmissive layer 779 may be formedby a highly transparent material (e.g., optically clear UV curable ink).Layer 779 may be provided, for example, for enhancing light extractionfrom light guiding sheet 10 or to promote adhesion of inner layer 777 tosurface 11. Transparent layer 779 should preferably have a refractiveindex that is about the same or greater than that of light guiding sheet10 to suppress TIR at the optical interface with surface 11. Transparentlayer 779 may also include a material that has enhanced adhesion tosurface 11 compared to the material of layer 777. According to oneembodiment, the adhesion of the material of transparent layer 779 tosurface 11 is greater than the adhesion of binder material 37 to surface11.

According to one embodiment, optically transmissive layer 779 mayinclude color filtering materials (e.g., colored ink). According to oneembodiment, optically transmissive layer 779 may include fluorescent orphosphorescent materials (e.g., phosphors) configured for convertinglight from a shorter wavelength to a longer wavelength. color filteringmaterials (e.g., colored ink).

According to one embodiment of a method of making wide-area illuminationsystem 900 in a front light configuration, layered light extractionfeatures 8 may be formed by a sequential deposition or reflective andopaque materials to the same discrete X and Y locations of surface 11.For example, light extraction features 8 may be formed by initiallyprinting a suitable pattern of microdots of a UV-curable white ink onsurface 11 in a first pass followed by printing the same pattern ofmicrodots of a UV-curable black ink on top of the white-ink microdots ina second pass. According to one embodiment, such overprinting may beperformed in succession without repositioning the substrate (lightguiding sheet 10) between the first and second passes. This can be done,for example, using a commercial inkjet printer that is capable ofprinting with different types/colors of ink simultaneously. Similarly,triple-layer light-extraction features 8 (such as that shown on thebottom of FIG. 13 ) may be printed sequentially in three passes of aprint head, without substrate repositioning between the printing passes.

FIG. 14 schematically depicts an embodiment of wide-area illuminationsystem 900 in a front-light configuration which is similar to that ofFIG. 13 except that light absorbing outer layers 778 conformably coatingan inner layer of light extraction features 8 are replaced with detachedlight blocking features 782 disposed in registration with reflectivelayers 777. Light blocking features 782 are formed on a thin transparentsubstrate sheet 781 covering surface 11. Light blocking features 782 maybe distributed over substrate sheet 781 according to the same twodimensional pattern as light extraction features 8. Furthermore, thepatterns of light blocking features 782 and light extraction features 8may be precisely aligned relatively to each other so that each lightblocking feature 782 provides a discrete opaque cover for the respectivelight extraction feature 8.

Substrate sheet 781 may be advantageously separated from light guidingsheet 10 by a thin spacing layer 784 to accommodate the thickness oflight extraction features 8 and to provide an air gap sufficient tomaintain TIR in sheet 10. Substrate sheet 781 may conventionally have afilm thickness which can be much less than the thickness of lightguiding sheet 10, e.g., by at least 2 times, 3 times, 5 times, 10 timesor more.

Each light blocking feature 782 is formed by an opaque material whichcan be light absorptive or reflective. It may be formed by the samematerials and using the same methods described in reference to outerlayers 778 of FIG. 13 . According to one embodiment, light blockingfeatures 782 may be deposited to a surface of substrate sheet 781 thatis facing light guiding sheet 10. According to one embodiment, lightblocking features 782 are deposited to a surface of substrate sheet 781that is facing away from light guiding sheet 10 (e.g., as illustrated inFIG. 14 ). Each light blocking feature 782 should be disposed inregistration with the respective light extraction feature 8 and shouldpreferably cover the entire area of light extraction feature 8 from theviewer.

According to one embodiment, the size of each light blocking feature 782is at least the same or larger than the size of the respective lightextraction feature 8. According to different embodiments, the size oflight blocking features 782 may be greater than the size of therespective light extraction features 8 by at least 10%, 20%, 30%, 50%,or 100%. According to different embodiments, the area of light blockingfeatures 782 may be greater than the area of the respective lightextraction features 8 by at least 10%, 20%, 30%, 50%, 2 times, 3 timesand 4 times.

According to one embodiment of a method of making wide-area illuminationsystem 900 in a front light configuration, the method includes a step ofdepositing a predetermined pattern of microdots of white, colored orfluorescent UV-curable ink (light extraction features 8) to a surface ofa light guiding substrate (e.g., surface 11 of light guiding sheet 10)with variable spacing, a step of covering the surface of the lightguiding substrate with a thin transparent substrate (which can beexemplified by substrate sheet 781) and a step of depositing the samepattern of microdots of a highly opaque (preferably black or reflective)ink to a surface of the thin transparent substrate (e.g., to form lightblocking features 782). The method may further include a step of bondingthe thin transparent substrate (substrate sheet 781) to the lightguiding substrate (light guiding sheet 10) at select locations (e.g., atedges or corners) to prevent shifting the substrates relatively to eachother and ensure that light blocking features 782 are kept inregistration with light extraction features 8 when in handling oroperation. At the same time, light extraction features 8 may beconfigured to provide an air gap between substrate sheet 781 and lightguiding sheet 10 and minimize optical cross-talk between them.

In operation, the embodiment of wide-area illumination system 900 ofFIG. 14 is similar to that of FIG. 13 except that forward-scatteredlight (as exemplified by ray segment 343) can escape from lightextraction features 8 and can propagate a relatively short distance awayfrom surface 11 and towards viewer 660 before it is blocked from furtherpropagation towards viewer 660 by light blocking features 782. Dependingon the type of the opaque material used for making light blockingfeatures 782 (e.g., absorbing or reflective), ray segment 343 may beabsorbed or reflected and recycled, similarly to several embodimentsdescribed above in reference to FIG. 13 .

According to an aspect, transparent substrate sheet 781 and a pattern oflight blocking features 782 form an opaque mask or overlay thatselectively blocks light emitted from areas of light guiding sheet 10corresponding to light extraction features 8. At the same time, thetransparent spacing areas between light blocking features 782 allow fora general unimpeded transverse light passage from image print 755 toviewer 660 and thus allow for a generally unimpeded viewing of imageprint 755. It may be appreciated that, with a proper alignment of themask or overlay and with the proper sizing of light blocking features782, the glare associated with individual semi-opaque light extractionfeatures 8 may be suppressed or even practically eliminated such thatthe apparent contrast for illuminated image print 755 may besignificantly enhanced compared to the case where no such mask oroverlay is used.

FIG. 15 schematically shows an embodiment of wide-area light guideillumination system 900 that has three planar light guides stacked onone another along the Z axis. Light guiding sheet 10 represents a firstplanar light guide, a light guiding sheet 20 represents a second planarlight guide, and a light guiding sheet 30 represents a third planarlight guide.

According to one embodiment, sheets 20 and 30 may be made from the samematerial as sheet 10. Sheets 20 and 30 may also have similar oridentical dimensions and structure as sheet 10. According to oneembodiment, each of sheets 10, 20 and 30 may differ from the other twosheets in one or more of the following: material, structure,composition, thickness, length and/or width dimensions, color, surfacetexture, and light extraction patterns.

Sheets 10, 20 and 30 are disposed in a close proximity to each other butare also separated from each other by a small air gap of the order ofseveral micrometers. The air gap may be provided, for example, toaccommodate the height of light extraction features and also to preventoptical contact between the surfaces of the sheets.

Sheet 10 has light extraction features 8 formed in surface 11 and lightextraction features 9 formed in surface 12. Sheet 20 has lightextraction features 91 formed in a broad-area surface 21 and lightextraction features 92 formed in an opposing broad-area surface 22.Sheet 30 has light extraction features 93 formed in a broad-area surface31 and light extraction features 94 formed in an opposing broad-areasurface 32.

According to one embodiment, substantially all of light extractionfeatures 8, 9, 91, 92, 93 and 94 are formed by microdots of a UV-curableink, such as white-color light scattering ink, colored ink, fluorescentink, or any combination of suck inks. The light guiding sheets arepatterned such that, when all three sheets are pressed against eachother, the light extraction features formed in one sheet can touch therespective surface of an adjacent sheet. For example, light extractionfeatures 9 can touch surface 21 and light extraction features 91 cantouch surface 12. In this case, the air gap between sheets 10 and 20 maybe primarily defined by the height (or thickness) of light extractionfeatures 9, 91, 92 and/or 93. As shown in FIG. 15 , at least some lightextraction features 9 may be disposed in spaces between light extractionfeatures 91 and at least some light extraction features 92 may bedisposed in spaces between light extraction features 93.

Three different arrays of LEDs are provided to independently illuminatelight guiding sheets 10, 20 and 30. Sheet 10 is illuminated by an arrayof LEDs 2 optically coupled to light input edge surface 13, sheet 20 isilluminated by an array of LEDs 2′ optically coupled to a light inputedge surface 96, and sheet 30 is illuminated by an array of LEDs 2″optically coupled to a light input edge surface 97.

According to one embodiment, LEDs 2, 2′ and 2″ may be configured to emitlight in different colors (e.g., LEDs 2 can be red, LEDs 2′ can begreen, and LEDs 2″ can be blue) such that the respective patterns ofsheets 10, 20 and 30 can emit light in the respective colors. Lightguide illumination system 900 of FIG. 15 can be configured toindependently emit light in three different colors simultaneously or ina succession. For example, according to one embodiment, light extractionfeatures 8 and 9 may cumulatively form a first pattern configured todisplay a first image when illuminated, light extraction features 91 and92 may cumulatively form a different second pattern configured todisplay a different second image when illuminated, and light extractionfeatures 93 and 94 may cumulatively form a different third patternconfigured to display a yet different third image when illuminated. LEDs2, 2′ and/or 2″ may be independently controlled and selectively turnedon and off to illuminate and display the first, second and/or thirdpattern or image, respectively. The relative intensity of light emittedfrom the respective patterns may be controlled by selectivelycontrolling the brightness of the arrays of LEDs 2, 2′ and/or 2″ (e.g.,by individual dimming).

According to one embodiment, light guide illumination system 900 of FIG.15 may be configured to emit light from both outermost surfaces 11 and32 and to be viewable from both sides. According to one embodiment, areflective sheet or surface may be provided on either side (e.g., atsurface 11) to limit light emission to one side only (e.g., surface 32).According to one embodiment, light extraction features 8, 9, 91, 92, 93and/or 94 may have different optical properties (e.g., different colorsof the ink used to form the respective light extraction features).Accordingly, when illuminated by LEDs 2, 2′ and/or 2″, system 900 may beconfigured to display two, three or more different patterns in differentcolors or intensity. According to one embodiment, all of the patternsmay be displayed simultaneously in an illuminated state by illuminatingsheets 10, 20 and 30 by respective LEDs 2, 2′ and 2″. According to oneembodiment, the patterns may be displayed one by one, e.g., by dimmingor turning LEDs 2, 2′ and 2″ on and off in a rapid succession, toproduce various conspicuous visual effects, including but not limited toflashing, fading or motion effects.

According to one embodiment, a single LED source (e.g., LED 2) may beused to illuminate all of the three light guiding sheets 10, 20 and 30.In this case, the LED source should preferably have a light emittingaperture that is equal to or slightly less than the combined thicknessof light guiding sheets 10, 20 and 30 but greater than a combinedthickness of any two of the sheets. According to one embodiment, the LEDsource may be configured to emit light in a single narrow color range(e.g., in blue color) and the light extraction patterns of differentlight guiding sheets 10, 20 and 30 may be configured to extract lightwith different optical effects, e.g., converting the LED light todifferent colors.

For example, referring to FIG. 15 , light extraction features 8 and/or 9may be configured to scatter a blue light without conversion, lightextraction features 91 and/or 92 may include a first fluorescentmaterial configured to convert the blue light into a first color, andlight extraction features 93 and/or 94 may include a second fluorescentmaterial configured to convert the blue light into a second color whichis different than the first color. Accordingly, wide-area light guideillumination system 900 of FIG. 15 may be configured to displaydifferent illuminated patters in different colors from the same widearea even when single-color LED light sources are used.

According to an alternative embodiment, the LED source illuminating allthree light emitting sheets 10, 20 and 30 may be configured to emit agenerally white light and the emission in different colors may beprovided by incorporating various color pigments into the respectivelight extraction features of different sheets. For example, lightextraction features 8 and/or 9 may include a blue or cyan pigment, lightextraction features 91 and/or 92 may include a red or magenta pigment,and light extraction features 93 and/or 94 may include a green or yellowpigment.

According to one embodiment, sheets 10, 20 and 30 may be positionedslightly father apart and optical sheets 71 may be inserted in thespaces between the spaced-apart sheets (FIG. 16 ). According to oneembodiment, each optical sheet 71 is a transmissive light diffusingsheet. The transmissive light diffusing sheets may be configured, forexample, to mask individual light extraction features or blur theoutlines or optical irregularities of the light extraction patterns.According to one embodiment, additional sheets 71 may also be providedon the sides of surfaces 11 and 32 and configured to diffuse lightemitted from those surfaces. Alternatively, or in addition to that, areflective surface may be provided on either side (at surfaces 11 or 32)to recycle light emitted from the respective surface (11 or 32) andcause emitting substantially all of the light extracted from sheets 10,20 and 30 through the opposite surface (32 or 11, respectively).

FIG. 17 schematically illustrates an embodiment of light guide 800including several different types of light extraction features formed inboth opposing broad-area surface 11 and 12. Light extraction features402 exemplify round dome-shaped, fully cured microdots of a UV inkformed on surfaces 11 and 12 of light guiding sheet 10. Light extractionfeatures 404 exemplify flat-top microdots of a UV ink. Each flat-topmicrodot may have a rounded trapezoidal shape having a constant ornear-constant thickness for at least 50%, 60%, 70%, 80% or 90% of itsarea.

Light extraction features 406 exemplify small spherically shaped bumps,protrusions or microlenses formed in surfaces 11 and 12. Lightextraction features 408 exemplify rounded dimples formed in surfaces 11and 12. Light extraction features 410 exemplify conical dimples ortriangular prismatic grooves formed in surfaces 11 and 12. Lightextraction features 412 exemplify conical or triangular prismaticprotrusions formed in surfaces 11 and 12.

Light extraction features 414 exemplify high-aspect-ratio cavities orgrooves formed in surfaces 11 and 12. By way of example and notlimitation, the cavities, dimples or grooves may be formed by any of thefollowing methods: molding (e.g., compression or injection molding),embossing (e.g., hot embossing), etching (e.g., chemical or ionbombardment), or laser ablation. According to one embodiment, sheet10/light guide 800 is made from an acrylic material and the cavities,dimples or grooves are formed using a CO₂ laser ablation. Lightextraction features 407 exemplify a continuous series of microstructuresor undulations formed surfaces 11 and 12. Such microstructures orundulations may include both cavities and protrusions formed in therespective surfaces and may be configured to scatter the extracted lightacross a wide range of directions.

A light extraction feature 416 exemplifies a compound light extractionstructure that is formed by light extraction feature 406 (sphericallyshaped bump, protrusion or microlens) overprinted with light extractionfeatures 402 formed by a round dome-shaped, fully cured microdot of a UVink that has approximately same or slightly larger dimensions as theunderlying light extraction feature 406 and that is conformably coatinglight extraction feature 406. A light extraction feature 418 exemplifiesa different compound light extraction structure that is formed by lightextraction feature 408 (rounded dimple in surface 11) overprinted withlight extraction features 402 which fills the respective dimple with alight scattering ink material and has approximately same (or slightlylarger) dimensions as the underlying dimple. A light extraction feature419 exemplifies a yet different compound light extraction structure thatis formed by light extraction feature 407 overprinted with flat-toplight extraction features 404 which overcoats the respective surfacemicrostructures or undulations formed in surface 12. The same principlesof forming compound light extraction structures may be applied withoutlimitations to other types light extraction features employing surfacestructures (e.g., such as those illustrated in FIG. 17 ). The compoundlight extraction structures may include several printed optical layersand may be advantageously used, for example, for enhancing the lightextraction efficiency or for adding further control to the angulardistribution of light emitted from light guide 800.

It is noted that FIG. 17 , illustrating a cross-section of light guide800, also exemplifies highly elongated (linear) geometricalconfigurations of light extraction features 402, 404, 406, 408, 410,412, and 414, which may have a longitudinal axis perpendicular to the YZplane (parallel to the X axis). The longitudinal lengths of the lightextraction features 402, 404, 406, 408, 410, 412, and 414 (in a linearconfiguration), as measured along the X axis, may vary in a broad range.According to one embodiment, the lengths of the respective lightextraction features may be at least two or three times of their widthmeasured along the Y axis but much less than the respective dimension ofsheet 10, as measured along the X axis. According to one embodiment, atleast some of linear light extraction features 402, 404, 406, 408, 410,412, and 414 may have lengths that approximates the respective dimensionof sheet 10, as measured along the X axis. According to one embodiment,linear light extraction features 402, 404, 406, 408, 410, 412, and 414may be oriented along a perpendicular direction, e.g., parallel to the Yaxis. According to one embodiment, linear light extraction features 402,404, 406, 408, 410, 412, and 414 may be oriented at an oblique anglewith respect to the X axis and/or Y axis.

It should be understood that the above teachings in reference to lightextraction features 402, 404, 406, 408, 410, 412, and 414 may beapplied, without limitations, to configuring any of light extractionfeatures 8, 9, 91, 92, 93, and 94 described in the preceding embodimentsand any of the light extraction features of the embodiments describedbelow. Furthermore, the different types or configurations of lightextraction features described above can be combined within the samelight guiding sheets (e.g., sheets 10, 20 and/or 30), for example, toachieve different visual effects (e.g., displaying different illuminatedpatterns and/or colors, statically or in succession) or to obtaindifferent predefined luminance distributions. For example, surface 11 oflight guiding sheet 10 may have micro-dots of white UV-curable inkhaving one composition (e.g., a first volumetric density of lightscattering particles within a transparent binder), micro-dots of whiteUV-curable ink having a different composition (e.g., a second volumetricdensity of light scattering particles within the transparent binder),and micro-cavities, dimples or micro-bumps distributed across thesurface according to a two-dimensional pattern. According to oneembodiment, light extraction features of one type or composition may beformed in surface 11 and light extraction features of a different typeor composition may be formed in opposite surface 12.

Furthermore, the teachings of embodiments described herein in referenceto light extraction features 8 or 9 may be applied, without limitations,to configuring any of light extraction features described in referenceto other embodiments of illumination system 900. According to oneembodiment, light extraction features 402, 404, 406, 407, 408, 410, 412,and 414 may be configured to overlap with one another, including thecase when the overlapping light extraction features are of a differenttype. For example, light extraction features 402 and/or 404 formed byprinted microdots of a UV ink may be disposed on top of light extractionfeatures 406, 407, 408, 410, 412 and/or 414 representing surface reliefstructures formed in surfaces 11 and/or 12 (e.g., by means ofoverprinting. When the bottom light extraction features representprotrusions in surface 11 or 12 (e.g., light extraction features 406,407 or 412), the overprint (e.g., light extraction features 402 and/or404) may conformably coat such protrusions. When the bottom lightextraction features include cavities in surface 11 or 12 (e.g., lightextraction features 407, 408, 410, 414), the overprint (e.g., lightextraction features 402 and/or 404) may be configured to seal therespective cavities and preserve the refractive or TIR opticalinterfaces of the cavities. Alternatively, the overprint may beconfigured to partially or completely fill the respective cavities.According to one embodiment, the volume of the overprint may be less orequal to the volume of the cavity. According to one embodiment, thevolume of the overprint may be greater than the volume of the cavitysuch that the respective overlapping light extraction featurescumulatively form a protrusion in surface 11 or 12.

FIG. 18 through FIG. 21 schematically show polar luminous intensitydistribution graphs calculated using optical raytracing for differentconfigurations and combinations of light extraction features formed inbroad-area surfaces of a thin planar light guide illuminated from twoopposite edges by arrays of LED sources, e.g., as illustrated by theexample of LEDs 2 optically coupled to edge surfaces 13 and 14 in FIG. 1. The sampling plane selected for each graph is perpendicular to theprevalent plane of the planar light guide and to both light input edges.In other words, the sampling plane illustratively corresponds to a planeparallel to the YZ plane in FIG. 1 when light is input through edgesurfaces 13 and 14 only. Assuming a horizontal orientation of the planarlight guide, the upper half of each graph corresponds to an upwardemission from the top surface of the light guide and the lower half ofeach graph corresponds to a downward emission from the bottom surface ofthe light guide.

By way of example and not limitation, the optical configurations ofwide-area light guide illuminations systems which angular emissiondistributions are illustrated in FIG. 18 through FIG. 20 may beadvantageously selected for direct/indirect illumination (e.g.,incorporated into a suspended lighting fixture) while the configurationcorresponding to FIG. 21 may be advantageously selected for direct-onlyillumination (e.g., incorporated into a recessed or surface-mountdownlight troffer).

FIG. 18 shows a symmetrical luminous intensity distribution in whichlight output from opposite top and bottom broad-area surfaces of thelight guide is near identical in terms of both the angular dependenceand the total energy of emitted light. The angular distribution patternsfor each side (e.g., top and bottom) of the light guide closelyapproximate that of a Lambertian (cosine) emission having an equivalentoverall light output.

FIG. 19 is a calculated polar luminous intensity distribution graph foran alternative exemplary configuration of light extraction featureswhich produce a substantially Lambertian emission from both the top andbottom surfaces with the overall light output from the bottom surfacebeing significantly greater than the overall light output from the topsurface. According to different embodiments, the light extractionpattern may be configured to produce a ratio between the peak luminanceof the top surface and the peak luminance of the bottom surface (e.g.,along a normal direction) which may take one of the following values (orany ranges in between): 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, 1, 1.1,1.2, 1.3, 1.4, 1.5, 1.6, 1.7, 1.8, 1.9, 2, 2.5, 3, 4, or 5.

FIG. 20 is a calculated polar luminous intensity distribution graph fora further alternative exemplary configuration of light extractionfeatures that produces two different angular emission distributions of a“batwing” type. The top surface produces a “batwing” angular emissionwith a wide throw and a small on-axis “bulge”. The bottom surfaceproduces a “batwing” angular emission with a much narrower throw andwith a reduced on-axis intensity.

FIG. 21 is a calculated polar luminous intensity distribution graph fora yet further alternative exemplary configuration of light extractionfeatures and for the case where a semi-specular reflective sheet isplaced on top of the top surface of the edge-lit light guide such thatthe light is emitted in a downward direction only. The resulting angularintensity distribution is directional with the intensity being nearconstant within a range of angles between about −50° to about +50° froma surface normal.

Example 2

A wide-area light guide illumination system was made using an edge-litplanar acrylic light guide having a thickness of about 1.5 mm and majordimensions of about 600 mm by 600 mm. The light guide was patterned onone side using microdots of a UV-cured white ink. The light guide wasilluminated from two opposing edges using two strips ofphosphor-converted white SMD LEDs positioned in a close proximity to therespective edge surfaces. The total light output of the two bare LEDstrips (without the light guide) was measured at 4,500 lumens.

The dimensions of the light emitting aperture of each LED were about 1.2mm by 1.2 mm. The light extraction pattern was produced by a randomizedtwo-dimensional array of microdots deposited to the light guide surfaceusing a commercial flatbed UV printing machine (a UV printer). Thedensity of the pattern was made gradually increasing from the lightinput edges towards the center of the light guide. Each light extractionfeature was represented by an individual microdot produced by a singledroplet of the UV ink. Each microdot had a size of approximately 100-130micrometers and a maximum thickness of about 6 micrometers in thecenter. The volume of each microdot was about 40,000 cubic micrometers.No light diffusing or reflective sheets were included into the wide-arealight guide illumination system of this example.

The luminous intensity distributions of the emission from the top andbottom surfaces were measured using a Type C Goniophotometer System. Thelight guide was oriented horizontally with the surface patterned withlight extraction features facing up. The results of the goniophotometricmeasurements are summarized in the annotated polar luminous intensitydistribution graph shown in FIG. 22 . The intensity units on the graphare candelas.

Referring to FIG. 22 , a curve 801 represents a measured angulardependence of the luminous intensity of the emission from the topsurface of the light guide in a first vertical plane (through horizontalangles 0°-180°). A curve 802 represents a measured angular dependence ofthe luminous intensity of the emission from the top surface in a second(orthogonal) vertical plane (through horizontal angles 90°-270°). Acurve 803 represents a calculated reference angular dependence of theluminous intensity for a Lambertian emitter of a similar total lightoutput compared to the top surface.

Referring further to FIG. 22 , a curve 804 represents a measured angulardependence of the luminous intensity of the emission from the bottomsurface of the light guide in a first vertical plane (through horizontalangles 0°-180°). A curve 805 represents a measured angular dependence ofthe luminous intensity of the emission from the bottom surface in asecond (orthogonal) vertical plane (through horizontal angles 90°-270°).A curve 806 represents a calculated reference angular dependence of theluminous intensity for a Lambertian emitter of a similar total lightoutput compared to the bottom surface.

As it can be seen from the graph, both the top and bottom surfacesproduced near-Lambertian angular intensity distributions with thedeviations from the “ideal” Labmertian distribution being about 10% orless for most observation angles. The total light output from thepatterned top surface was about 10% greater than that from thenon-patterned bottom surface.

Example 3

The wide-area light guide illumination system described the Example 2was modified by adding an opaque diffuse reflector above the top surfaceof the planar light guide and measured using the same procedure. Themeasurement results are summarized in the annotated polar luminousintensity distribution graph shown in FIG. 23 . Referring to FIG. 23 ,the total light output from the bottom surface approximately doubledcompared to the Example 2 as a result of light reflection from theopaque diffuse reflector. Furthermore, the measured emission in bothorthogonal planes became even more closely resembling the “ideal”Lambertian emission normalized to the same total light output. Thedeviation between the measured intensity and the calculated intensitybased on the Lambertian law constituted 5% or less for most measuredangles. Due to the high opacity of the top reflector (˜100%), virtuallyno light was emitted from the top surface of the device.

Example 4

FIG. 24 shows an annotated photograph of an ordered light extractionpattern of semi-opaque microdots printed on a broad-area surface(exemplifying surface 11) of a planar acrylic sheet (exemplifying lightguiding sheet 10). Each of the printed microdots formed a discrete lightextraction feature on the surface (exemplifying individual lightextraction feature 8). The light extraction pattern was printed using asemi-opaque UV-curable white ink containing TiO₂ particles inconcentrations from 5% to 15% by weight. The printed pattern had a fixedpitch in both X and Y directions and included microdots of differentsizes. At least some of the larger light extraction features 8 wereformed by individual microdrops of the UV ink each having a volume ofaround 40,000 cubic micrometers.

Example 5

FIG. 25 shows an annotated photograph exemplifying a random pattern oflight extraction features 8 printed using the same type of ink andprinting hardware as in the Example 4. The resulting printed pattern ofFIG. 25 included a random mix of different shapes (including regular,quasi-regular, round, elongated, irregular and highly irregular shapes),orientation and sizes.

Example 6

FIG. 26 shows an annotated photograph of an ordered pattern of opticallyclear microdots (transparent light extraction features 8) printed on asurface of light guiding sheet 10 using a transparent UV-curable ink.The microdots in this print had elongated shapes generally alignedparallel to the X axis.

Example 7

Similarly, FIG. 27 shows an annotated photograph of optically clearmicrodots (transparent light extraction features 8) that were printed ata different location of the same substrate (light guiding sheet 10)using the same type of ink as in the Example 6 but using a lower densityof the pattern (greater spacing distances SPD between adjacentmicrodots).

Example 8

FIG. 28 shows an annotated three-dimensional photograph (obtained usingan imaging 3D microscope/optical surface profiler) of individual lightextraction feature 8 formed by a microdot of semi-opaque, UV-curablewhite ink. The microdot had a shallow spherical shape with asubstantially round, regular outline. The diameter of the shallowdome-shaped microdot was about 130 micrometers and the maximum thicknessat the center was about 8 micrometers.

Example 9

FIG. 29 illustrates an individual printed microdot (light extractionfeature 8) which is similar in size and composition to that of theExample 8, except that is has an irregular, elongated shape withsomewhat fuzzy borders at the longitudinal ends of the elongated shape.

Example 10

A planar acrylic light guide (exemplifying light guiding sheet 10) waspatterned for light extraction using a two-dimensional randomizedpattern of microdots formed by a white-color UV-curable ink includinglight-scattering nanoparticles suspended in a clear binder material. Themicrodots were deposited to one of the broad-area surfaces of the lightguide using a different commercial UV printing machine and using adifferent printing regime compared to the previous Examples. The overalllight extraction pattern included areas of different distributiondensity of the microdots (sub-patterns). A series of photographs ofdifferent sampling areas of the patterned surface corresponding todifferent distribution densities of the microdots was taken using amicroscope camera. The resulting photographs, enumerated a) through g),are shown in FIG. 30 . The field of view of each photograph isapproximately 3.3 mm by 2.5 mm.

Photograph a) shows relatively sparsely populated microdots(representing individual light extraction features 8) which correspondto an area of surface 11 that is relatively close to a light input edge(not shown). The microdot patterns within same-sized sampling areasgradually become denser with the increase of the distance from the lightinput edge, as shown in photographs b) through d), but still withoutsignificant overlaps of individual microdots.

Photograph e) illustrates a sampling area that is located even furtheraway from light input edge and has an even greater density of themicrodots (with lower spacing distances SPD) compared to the samplingareas depicted on photographs a) through d). As it can be seen fromphotograph e), at least some of the adjacent microdots partially overlapone another.

Photograph f) illustrates a sampling area located even further away fromlight input edge and having an even greater distribution density of themicrodots. There is also a substantial amount of overlap betweenadjacent microdots (having less-than-zero separation distances SED),including groups of 2, 3 or more overlapping microdots. Photograph g)depicts a sampling area characterized by yet even greater density of themicrodots which overlap in large groups and create a continuous,randomly textured three-dimensional light extraction surface withoccasional random voids 155. Voids 155 were substantially free from thelight scattering material or contained a substantially reduced amount ofthe light scattering material compared to the adjacent surface texturesproduced by the fully loaded light scattering ink material.

FIG. 31 shows a composite three-dimensional image of an individualprinted microdot (exemplifying individual light extraction feature 8),which shape and dimensions were typical for at least some portions ofthe light extraction pattern produced in this Example. The image wasobtained using a 3D surface profiler/microscope from Zeta Instruments(model Zeta-200). FIG. 32 shows a cross-sectional surface profile (acurve 500) of the printed microdot taken along a cutting plane 99passing through a mid-section of the microdot.

As can be seen from FIG. 31 and FIG. 32 , the exemplary printed microdothas a generally round shape with a “flat-top” (truncated cone) 3Dgeometry, a narrow (<5 micrometers) outer rim with a relatively low(<0.2-0.3 microns) thickness, sloped side walls (visualized as walls 502of curve 500), a diameter between 70 and 80 micrometers, and anear-constant thickness between 1.2 and 1.5 micrometers in the areabetween the sloped side walls. The volume of the exemplary printedmicrodot (in the fully cured, solid state) was measured at about 4,000cubic micrometers.

As demonstrated by surface profile curve 500, the surface of theexemplary microdot had random surface irregularities 504 on a scale ofthe order of 0.1 micrometer. The random surface irregularities can alsobe seen in the form of a visible matte-finish surface texture on themicrophotograph of FIG. 31 . Somewhat similar surface irregularities mayalso be noted for light extraction feature 8 depicted in FIG. 28 .According to an aspect, the exemplary printed microdot represented athin, semi-opaque layer of a light scattering material (white ink)having a near-constant thickness of the order of one micrometer andoccupying a round area having a diameter of less than 80 micrometers.

Example 11

A thin and flexible sheet of highly transparent plastic material (whichwas subsequently used as a planar light guide in an edge-litillumination system) was patterned for light extraction using atwo-dimensional randomized pattern of microdots. The microdots wereformed by a light-scattering, non-absorbing, white-color UV-curable inkincluding light-scattering nanoparticles suspended in an optically clearbinder material. The binder material has a refractive index of about1.52. The microdots were deposited to one of the broad-area surfaces ofthe thin and flexible sheet (light guide) using a yet differentcommercial UV printing machine and using a yet different printing regimecompared to the previous Examples. Each microdot was formed by anindividual drop of the UV ink which was instantly cured (within afraction of a second) upon printing to the sheet surface. An LED UV lampoperably attached to the print head was used for UV ink curing. The UVink was recirculated within a closed-path fluid recirculation circuitwithin the UV printing machine and heated to about 40-45° C. within theprint head. The volume of each ink drop (microdrop) was selected atapproximately 15-20 picoliters. Upon curing, each printed microdotformed a solid, round-shape light extraction surface feature/structurehaving a volume of about 15,000-20,000 cubic micrometers and a diameterof about 100 micrometers.

The density of the printed pattern was gradually increased with adistance from the light input edge such that at least some microdotsformed at the opposite edge overlapped on one another. The densitygradient was calculated using computer raytracing and optical modelingconfigured to identify a density distribution which would produce arelatively uniform emission pattern. FIG. 41 shows the photograph of a3.3 mm×2.5 mm sampling area of the resulting printed pattern of lightextraction surface features/structures.

The light guide retained a perceptibly high degree of transparency afterit was patterned. For example, objects could clearly be seen through thepatterned light guide when it was not illuminated from the inside. Whenilluminated by a series of LEDs optically coupled to an edge of thesheet, the resulting printing-patterned light guide produced a highlyuniform emission with the intensity variations of surface emission ofabout 5% or less from the average intensity. The emission wasdistributed between the sides of the light guide approximately 55%/45%proportions. The angular distribution of the emission wasnearly-Lambertian from each side of the light guide.

End of Example 11

FIG. 33 shows an embodiment of light guide illumination system 900having light extraction features of different types and composition. Alight extraction feature 602 is formed by a layered surface structureincluding layers 611, 612, 613 and 614 disposed on top of each other.Each outer layer may be conformably coating the respective layerdisposed below. Layer 611 is formed by an optically transmissiveUV-curable color ink (e.g., red, green, blue, yellow, magenta, or cyan)having a refractive index which is equal to or greater than therefractive index of light guiding sheet 10. Layer 612 is formed by aUV-curable light scattering ink provided at a thickness such that theresulting layer is semi-opaque and substantially non-absorbing. Layer613 is formed by an optically clear adhesive material (e.g., UV-curableclear primer ink). Outer layer 614 is formed by a highly opaquereflective material (e.g., UV-curable metallic ink or aluminum foil)conformably coating the inner layers. A further protective layer (e.g.,clear lacquer or colored ink) may be further deposited on top of layer614. At least some or all layers forming light extraction feature 602may have elastic or viscoelastic properties even when fully cured. Suchproperties may be advantageously selected, for example to supportrepeated flexing of the patterned portions of sheet 10. Sheet 10 of FIG.33 may be illuminated from one or more edges using LEDs 2 (not shown)emitting white light.

According to one embodiment of a method of making light guideillumination system 900, light extraction feature 602 may be formed bysequential depositing the respective layers to surface 11 usingUV-curable ink with instant UV curing. Multiple print passes may also beused, for example, to enlarge the area and/or thickness of the color inkon surface 11. For example, layer 611 may be formed by printing one ormore drops of a color ink directly to surface 11. This can be done usinga single printing pass or multiple passes. Each ink drop can bedeposited to approximately the same location of surface 11.Alternatively, the drops can be deposited with some offset relatively toeach other to cover a larger area of surface 11. Each individual drop ofthe ink may be partially or completely cured between the print passes.In an alternative, multiple drops of color ink may be overprinted on topof each other and then the resulting volume can be cured at once.

Similarly, layer 612 may be formed by overprinting layer 611 with alight scattering (e.g., high-reflectance white) UV ink such that theoverprinted area closely approximates the area of layer 611, which mayrequire multiple drops of ink (multiple print passages) or even a verylarge number of drops (e.g., more than 10 drops, more than 50 drops,more than 100 drops), depending on the desired size of light extractionfeature 602 and a minimum desired thickness of the light scattering inkmaterial. Adhesive layer 613 may be overprinted on top of lightscattering layer 612 to promote the adhesion of opaque reflective layer614 to layer 612 and/or surface 11.

Surface 11 may include any number of individual light extractionfeatures 602. The size of individual light extraction features 602 mayvary from several tens of microns to several centimeters. According tosome embodiments, the area of light extraction feature 602 (and theareas of respective layers 611, 612, 613 and 614) may be relativelylarge (e.g., much larger than the area of an individual printed drop).For example, one light extraction feature 602 may represent anindividual letter or distinctly visible image segment in an illuminatedsign. According to one embodiment, individual light extraction features602 may have sizes exceeding the thickness of sheet 10 by 2 times, 5times, 10 times or more.

When the light guiding substrate of sheet 10 is made from certain typesof glass or plastics that have a relatively low surface energy, the areaof surface 11 may be selectively coated using a primer ink layer (notshown) before depositing layer 611, to promote adhesion and/or scratchresistance. By way of example, the same adhesive ink material that isused for forming layer 613 may also be used as primer/adhesive layerunderneath layer 611. According to one embodiment, a support layer ofuncured or partially cured UV ink having strong adhesive properties maybe deposited to surface 11 before depositing layer 611 on top of it. Thesupport layer may be subsequently cured to a solid form such that layer611 permanently bonds to the support layer.

According to one embodiment, optically clear adhesive primer ink (whichcan be exemplified by layer 613) may also be provided between any of theoptical layers of light extraction feature 8 (e.g., between layers 611and 612). For example, layer 613 may be overprinted (with instant UVcure) on top of layer 611 before overprinting layer 611 with layer 612.According to one embodiment, adhesive/primer layer 613 may be providedbetween each of several coats of colored ink material to promotesub-layer to sub-layer adhesion (e.g. when forming layer 611 or layer612 using several print passes to build the required layer thickness).

According to one embodiment, an additional encapsulation layer ofscratch resistant ink or lacquer (which can be clear or opaque) can bedeposited on top of opaque layer 614. The encapsulation layer may beconfigured to shield the inner layers from the environment (e.g., airand/or moisture) and prevent or slow down the wear and/or oxidation ofopaque layer 614 or provide additional protection for photoactive layersunderneath it (e.g., layer 611). According to one embodiment, theencapsulation layer may be used in place of layer 614, in which case therespective light extraction feature(s) may be configured to emit lightinto opposite directions from surfaces 11 and 12.

According to one embodiment of a method of forming layered lightextraction feature 602, each its individual layer may be formed using aninstant UV curing process such that the next layer is deposited on apartially or fully cured bottom layer. It may be preferred that the UVlight (which can be exemplified by a LED lamp or a series of LEDpackages arranged into a strip or two-dimensional array and emitting UVlight in a 365-396 nm wavelength range) it attached to a movable printhead such that the drops of ink are cured immediately after beingdeposited to the surface. It may further be preferred that the intensityof UV source can be controlled to adjust the speed of curing. Accordingto one embodiment, the UV source may be configured to provide UV lightintensity of 1 W/cm² or less. According to one embodiment, the UV sourcemay be configured to provide UV light intensity of between 1 W/cm² and100 W/cm² in gradual increments.

It is particularly recognized that, unlike conventional UV digitalprinting of images on various substrates), precisely controlling theprocess of forming and curing of each individual ink drop on surface 11can be critical for the operation of system 900. For example, in aconventional image print produced using a UV printer, the dimensions ofindividual drops of UV curable ink may not be as important for theintended purpose (e.g., for visual perception of the image print as awhole), in part, due to the smallness of the individual ink dropscompared to the size of the image or macroscopic image features.However, the dimensions and shape (including, for example, thesize/diameter/area, roundness/elongation, thickness, and cross-sectionalprofile) of individual ink drops deposited to surface 11 of lightguiding sheet 10 may be critical for the operation and effectiveness ofillumination system 900.

It may be appreciated that the rate of light extraction from lightguiding sheet 10 is generally proportional to R², where R is acharacteristic radius of the light extraction feature 602. Accordingly,when each light extraction feature 602 (or light extraction feature 8,referring to other previously described embodiments) is formed by asingle drop of light-scattering ink, even relatively small systematicdeviation in drop size can result in significant differences in emissiondistribution from the surface of sheet 10. For example, this can makesystem 900 designed for uniform surface emission to appear non-uniformand can also affect the overall brightness and efficiency or result inenergy loss. Furthermore, significant variations in the thickness ofsome optically active layers (e.g., layer 611) may also notably affecttheir operation, including, for example, the rate of color filtering orcolor conversion (e.g., when layer 611 contains colored or luminescentink material). Thus, precisely controlling the dimensions and profile ofthe individual ink drops may be important and even critical at least forsome embodiments of illumination system 900.

It is further recognized that the dimensions and cross-section profileof individual ink drops may be defined, among other factors, by the timeelapsed between the deposition of the drop to surface 11 and exposingthe drop to UV light. For example, it was found that longer delays tendto result in greater diameters and lower thicknesses of the fully curedink drops, thus also resulting in larger-area and thinner lightextraction features compared to shorter delays between drop placement tosurface 11 and its curing. In view of this, the timing before UVexposure, as well as the UV light intensity and duration may beadvantageously selected to result in forming ink drops of prescribeddimensions (e.g., according to the examples described above).

According to one embodiment, a time period T_(s) between the depositionof each ink drop and beginning of UV exposure is less than 1 second.According to alternative embodiments T_(s) is less than 0.5 s, less than0.3 s, less than 0.2 s, and equal to or less than 0.1 s. This can beachieved by positioning a UV lamp at a distance from the print headwhich can be calculated based on the motion velocity of the print head.According to one embodiment, T_(s) is about zero (practically no delay).This can be achieved, for example, by positioning a UV source very closeto the print head (e.g., within several mm) or exposing the respectivearea of surface 11 to UV light at the time of ink jetting. At the sametime, it may be preferred that the ink jetting area of the print headitself is shielded from the UV light to prevent premature ink dropcuring and/or clogging the ink passages or nozzles within the printhead.According to one embodiment, T_(s) can be made greater than 1 s, greaterthan 10 s, and even greater than 1 minute or so, e.g., for allowing theink drop to spread to a prescribed size/diameter before curing ordepositing a next drop. According to one embodiment, light extractionfeature 602 (or its individual layers) may be formed by multiple inkdrops that are only partially cured, e.g., to promote adhesion betweenlayers or preventing excessive flow of the upper layer(s).

In operation, referring to FIG. 33 , a light ray 351 exemplifying lightpropagating in sheet 10 in a waveguide mode strikes an area of surface11 underneath light extraction feature 602. The refractive index oflayer 611 is preferably at least equal to or greater than the refractiveindex of the material of sheet 10 such that substantially all of thelight entering onto the respective optical interface is extracted fromsheet 10 and enters layer 611. Ray 251 further propagates through thebulk of color ink forming layer 611 and changes its color (e.g., due tofiltering out unwanted colors by the material of layer 611).

According to one embodiment, the ink used to form layer 611 may containcolor pigments that are suspended in a clear binder and provides someforward scattering. On the other hand, the ink may be configured tominimize back scattering. Furthermore, layer 611 may be provided at arelatively low thickness such that most of the light incident onto thelayer is transmitted to light scattering layer 612 without appreciableabsorption. According to one embodiment, the material of layer 611 maybe substantially transparent at least at the thicknesses in which it isused to form layer 611. Layer 612 is configured to efficiently scatterlight into all directions (e.g., using non-absorbing TiO₂ nanoparticlesand light diffraction as the primary light scattering mechanism. Layer612 is further configured to direct a first portion of the scatteredlight to back to layer 611 and direct a second portion of the scatteredlight to reflective layer 614. The first portion of the scattered lightis propagated through layer 611 for the second time and emitted fromsurface 12 towards viewer/observer 660 such that the area of lightextraction feature 602 can be conspicuously seen in the desired color(which is defined by the color of the ink used for layer 611). Thesecond portion of the scattered light is diffusely reflected from layer614 and is also directed towards viewer 660 at it passes through thebulk of layers 613, 612, 611 and sheet 10, contributing to the visualbrightness and conspicuity of light extraction feature 602.

A light extraction feature 606 has the same basic structure, compositionand operation as light extraction feature 602 except that layer 611 isformed by a UV ink having a different-color than that of lightextraction feature 602. A light extraction feature 604 includes lightscattering layer 612 and opaque reflective layer 614, with no color ink,and is configured to extract white light (or whatever color of lightemitted by LEDs 2) from sheet 10 without perceptible color change. Lightextraction features 602, 604, and 606 may be distributed over the areaof surface 11 in a pattern that creates a desired visual impression intwo, three or more different colors. According to one embodiment, atwo-dimensional pattern of multiple light scattering/non-colored lightextraction features 604 may be used in conjunctions with two-dimensionalpatterns of light scattering/colored light extraction features 602 and606. All these three patterns may overlap such that light extractionfeatures 602, 604 and 606 alternate with one along longitudinal andlateral dimensions of surface 11. The areal density (and, hence, spacingbetween individual features) of each pattern may be controlledindependently from each other such that the color or color tint of thesurface emission at certain locations of surface 12 can be differentfrom other locations of the surface.

Additional light extraction features 602 or 606 may be configured toinclude other colors (in any suitable number or colors, hue and/orintensity). Furthermore, system 900 FIG. 33 may be configured to includemonochromatic or narrow-band light sources (e. g, blue LEDs or RGB LEDs)and the color ink of layers 611 in at least some light extractionfeatures 602 or 606 may be replaced with a fluorescent ink. Suitableexamples of commercially available fluorescent inks may include but arenot limited 32530-series (fluorescent orange), 32550-series (fluorescentred), 32590-series (fluorescent green), and 32600-series (fluorescentmagenta) sold by Nazdar Ink Technologies. According to one embodiment, acustom-color fluorescent ink may be made by mixing a clear UV curableink material with a powder of microscopic fluorescent particles (e.g.,phosphors or quantum dots). Suitable examples of such fluorescentpowders include but are not limited to yellow, orange, green and redphosphors commercially available from PhosphorTech Corporation indifferent particles sizes and bandgaps. It may be appreciated that someof the high-efficiency phosphors have particle sizes in the range from10 to 20 micrometers and using smaller-size particles may degrade theconversion efficiency due to charge carrier surface recombination andother effects. On the other hand, it was found that efficientpiezoelectric ink jetting of picotiter-scale microdrops generallyrequires particle sizes much less than 1 micrometer, and preferablybelow 0.1 micrometers. Accordingly, when phosphors are used to makefluorescent ink, according to some embodiments, it may be critical toselect a proper size of the phosphor particles to facilitate ink jettingand yet maintain acceptable light conversion efficiency of the phosphormaterial. According to one embodiment, the size of the phosphorparticles is between 0.1 micrometers and 1 micrometer. According to oneembodiment, the size of the phosphor particles is about 0.5 micrometers.According to one embodiment, the size of the phosphor particles is about0.2 micrometers. According to one embodiment, the size of the phosphorparticles is about 0.1 micrometers. According to one embodiment,phosphors or other types of fluorescent/luminescent materials (e.g.,quantum dots) with particle sizes below 0.1 micrometers may be used.

A light extraction feature 608 is formed by a layered surface structureincluding layers 611, and 612 disposed on top of each other. Layer 611is formed by a color UV ink. Layer 612 is formed by a light scattering(e.g., white-color) ink. A light ray 352 exemplifies light which isextracted from light guiding sheet 10 by light extraction feature 608.Layer 612 is is provided at a thickness that causes a first portion oflight ray 352 entering layer 612 to pass through light extractionfeature 608 and a second portion of light ray 352 to beredirected/reflected back to light guiding sheet 10. The transmittedportion of light may be at least partially colored by layer 611 and maytherefore be emitted towards a viewer 661 in that color. Likewise, thereflected light portion will pass through layer 611 at least twice andwill also be colored. Depending on the angle of the re-entrance intosheet 10, the reflected light rays may be continue propagating in lightguiding sheet 10 or may be emitted towards viewer 660, as illustrated inFIG. 33 . Accordingly, wide-area light guide illumination system 900 maybe configured to emit light from both opposing surfaces 10 and 11 usinga large number of light extraction feature 608 distributed over surface11.

The thickness of layers 611 and 612 may be varied in a broad range. Forexample, for some light extraction features 608, layer 611 may have arelatively low thickness to provide a subtle coloring effect (low colorsaturation). In contrast, for some light extraction features 608, layer611 may have a relatively high thickness to provide a highly saturatedcolor. This may be achieved by printing the respective layers 611 usingmultiple passes. The thickness of layer 612 may be likewise controlledto provide different reflectance and transmittance characteristics forlight extraction features 608. For example, one light extraction feature608 may be provided with relatively thin layer 612 and configured toreflect and transmit about equal amounts of the incident light, whileanother light extraction feature 608 may be provided with relativelythick layer 612 and configured to reflect at least two times more lightthan is transmitted.

Light extraction feature 602, 604, 608, and/or 608 may be arranged intovarious two-dimensional patterns to display lines, dots, geometricalshapes, images, letters, and the like. Layers 611 of different lightextraction features 602, 604, 608, and/or 608 may be configured tofilter or convert different wavelengths causing different areas ofsurfaces 11 and/or 12 to emit light in different colors. According to anaspect, light extraction features 602, 604, 608, and/or 608 may be usedas pixels to form various patterns that emit light in multiple colorswhen illuminated by LEDs 2 emitting a white color (e.g., usingcolor-converting or RGB LED packages). One or more light extractionfeatures may include fluorescent inks that have different bandgaps andare configured to convert light to different colors. Two or more primecolors may be mixed within a single light extraction feature (e.g., bymixing pigments of different colors or fluorescent materials havingdifferent bandgaps).

FIG. 34 illustrates exemplary randomized patterns of light extractionfeatures 602, 604, 606, and 608 formed in surface 11 of sheet 10,according to one embodiment, showing several discrete/spaced-apart andseveral overlapping individual light extraction features, some of thelight extraction features forming discrete clusters. According to oneembodiment, the clusters of overlapping light extraction features 602,604, 606, and 608 may form relatively large areas with a high fillfactor (which can be defined as a ratio between the projected cumulativearea of light extraction features and the total area outlining therespective cluster) and may also form various image objects such asletters or geometrical shapes, for example. According to one embodiment,the fill factor or the density of respective density of light extractionfeatures 602, 604, 606 and 608 may be made gradually increasing with thedistance from light input edges/LEDs 2 (e.g., towards an opposite edgefor single-edge light input or towards the mid-portion of sheet 10 forlight input through multiple edges).

FIG. 35 schematically depicts an embodiment of wide area waveguideillumination system 900 in an edge-lit illuminated sign implementationin which each letter of the word “OPEN” emits light in a different coloraccording to the principles discussed above. In a non-limiting example,the illuminated sign may be configured such that the letters “O”, P”,“E”, and “N” emit light in red, green, cyan and magenta colors,respectively. According to one embodiment, area portions or edges of theletters “O”, P”, “E”, and “N” facing the light input edges of sheet 10may have a lower density of light extraction features 602, 604, 606,and/or 608 compared to the inner area portions of the letters. Thedensity may be particularly varied to create an approximately constantrate of light extraction from each portion of patterned areas 55. Forexample, it may be appreciated that an area 131 of the letter “E” inFIG. 35 may receive less light that an area 130 of the same letter whenthe density of light extraction features is constant. Accordingly, theaverage density of light extraction features within area 130 may be madeat least 2 times, at least 3 times, at least 4 times, at least 5 times,at least 6 times, at least 10 times greater than the average density oflight extraction features within area 131 such that the luminance orvisual brightness of areas 130 and 131 is approximately the same or thedifference is within a desired range (e.g., within 10%, within 30%,within 50%, within 100% or within 150%.

FIG. 36 schematically depicts an embodiment of illumination system 900in which light extraction features 8 are formed from an adhesive inkmaterial and used to bond light guiding sheet 10 to a reflector 370.Reflector 370 may be exemplified by an opaque film-thickness sheet ofdiffusely reflecting material having high hemispherical reflectance,preferably above 90%. Light extraction features 8 are configured at asufficient thickness such that the bonded structure creates air gaps 376between sheet 10 and reflector 370. Air gaps 376 ensure the TIRoperation of sheet 10. The adhesive ink material preferably has arefractive index that is about the same or greater than the refractiveindex of the material of sheet 10 to completely suppress TIR at therespective locations. According to one embodiment, the adhesive inkmaterial is optically clear. According to one embodiment, the ink maycontain light scattering particles, one or more color pigments or anycombination thereof. The material of the ink preferably has a high tackstrength so that it bonds strongly to both surface 11 of sheet 10 and areflective surface 372 of reflector 370.

In operation, a light ray 361, propagating in sheet 10 using opticaltransmission and TIR, is extracted by light extraction feature 8 and isdiffusively reflected from surface 372 such that a significant fractionof the reflected light exits from surface 12 of sheet 10 (e.g., towardsviewer/observer 660). A parallel light ray 362 that misses lightextraction feature 8 is reflected from surface 11 and continues topropagate in sheet 10 in a waveguide mode.

The structure of system 900 depicted in FIG. 36 may be formed byapplying (e.g., laminating using a roll laminator, vacuum laminator, orplate press) reflector 370 to surface 11 having uncured orpartially-cured printed drops (or larger printed areas includingmultiple drops) of a UV curable adhesive primer ink. The partial curingmay be achieved, for example, by exposing the drops to a shorter curingtime or lower UV intensity than would otherwise be needed for fullycuring the UV ink. The UV exposure may be adjusted such that theviscosity of the ink material significantly increases (compared to thelow-viscosity jettable ink fluid) and prevents excessive flow and suchthat the ink material remains soft and tacky to enable adhesion to othersurfaces. Once the adhesive primer ink sufficiently wets surface 372, itcan be fully cured to complete the adhesion process (e.g., byilluminating surface 12 using a UV light source). For this process, itmay be preferred that the material of sheet 10 has sufficienttransmittance in the respective UV light spectrum.

According to one embodiment, reflector 370 may be formed from a thin,stretchable and compressible material and is configured to bend togetherwith sheet 10 while maintaining a strong bond to surface 11 (using lightextraction features 8). For this purpose, it may also be preferred thatthe material of light extraction features 8 remains flexible and elasticafter the full cure. According to some embodiments, reflector 370 may beformed by a transparent sheet or plate having a reflective surface 374.Reflective surface 374 may be exemplified by a reflective coating onsurface 374, which can be of a specular or diffuse type. Reflectivesurface 374 may also be exemplified by an image print or indicia havingareas that reflect more light than the areas disposed in between.According to one embodiment, the thickness of the transparent body/layerforming a part of the reflector 370 may be configured at a thicknessthat is equal to or greater than one half of the average spacing betweenlight extraction features 8. This configuration may be advantageouslyselected, for example, to enhance mixing of the light beams produced byindividual light extraction features 8 on surface 374 and thus enhancethe uniformity of the illumination of surface 374. According to oneembodiment, reflector 370 may be replaced by a thin sheet of anoptically transmissive material, which may also be configured to diffuselight. In this case, system 900 may be configured to emit at least aportion of light from the surface of such thin sheet towards an oppositedirection from viewer/observer 660.

FIG. 37 schematically depicts an exemplary method/process and anapparatus 950 for forming light extraction features 8 (and/or lightextraction features 9, 602, 604, 606 and 608) discussed above or anytheir layers) on surface 11 using a UV curable ink. Apparatus 950includes a print head 832 configured for jetting (e.g., usingpiezoelectric mechanism) individual drops 79 of UV-curable ink whilescanning surface 11 along a direction 831 with a constant velocity. A UVlamp 834 is mounted to print head 832 using a spacer 838. Spacer 838,together with the velocity of the print head, defines time T_(s) betweenthe deposition of each drop 79 and irradiating the deposited drop by UVlight (which is exemplified by light rays 835) illuminating surface 11.Making spacer 838 longer extends time T_(s), allowing drops 79 to flowto a larger diameter on surface 11 before being cured by UV rays 835.Conversely, shortening spacer 838 longer shortens time T_(s) and limitsthe size of cured drops 79, which form individual light extractionfeatures 8, and also maximizes their thickness. Additionally, theintensity of UV light may be adjusted to increase or decrease the rateof curing.

According to one embodiment, apparatus 950 may be configured for adelayed curing, in which the light extraction pattern is printed in afirst pass, with UV lamp 834 turned off, and cured in a second pass withthe UV lamp turned on. The delay between the first and second passes canbe defined by time T_(s), e.g., calculated based on the desiredsize/thickness of light extraction features 8. Additional layers oflight extraction features 8 may be formed by depositing additional drops79 to the same locations of surface 11, without repositioning the lightguiding substrate. The ink-jetting passages and nozzles of print head832 should preferably be shaded from the UV lamp 834 to preventpremature curing and clogging the passages or nozzles. According to oneembodiment, UV lamp 834 may be set to a relatively low intensity suchthat each layer is cured only partially in a single pass. It was foundthat instant or delayed partial curing may sometimes promote thelayer-to-layer adhesion and allow for additional control over theuniformity of the printed layer compared to the full cure, resulting inwell-defined shapes of light extraction features 8. A stack of partiallycured layers can be fully cured after the process of forming lightextraction features 8 is complete. This can be done, for example, byscanning UV lamp 834 over the printed areas.

FIG. 38 shows a schematic graph showing exemplary dependencies of thediameter and thickness of UV printed light extraction features 8depending on the curing delay time T_(s). This graph may be particularlyrepresentative of the case where each light extraction feature 8 isformed by a single drop of UV curable ink. However, the same or similarprinciples may generally apply to cases where each light extractionfeature is formed by multiple drops printed adjacent to each other oroverprinted on top of each other.

The diameter of light extraction feature 8 is represented by a curve1010 and the thickness of light extraction feature 8 is represented by acurve 1012. Points 1014 and 1016 represent an exemplary targetcombination of the diameter and thickness at an optimum curing delay.Minimum diameter Dia_(min) and maximum thickness Thickn_(max) correspondto a minimum value of delay time T_(s), (T_(s min)), which may representa “true” instant curing regime, e.g., when the respective ink drop isexposed to UV light immediately upon contact with surface 11. Maximumdiameter Dia_(max) and minimum thickness Thickn_(min) correspond to amaximum value of delay time T_(s), (T_(s min)), which may be defined,for example, based on the operator's choice and the desired opticalproperties of light extraction features 8.

As it can be seen, the optimum value of delay time T_(s) corresponds tofairly steep portions of curves 1010 and 1012, where both the diameterand thickness of light extraction feature 8 are quite sensitive tovariations in the delay time. Accordingly, the rate of light extraction,which is proportional to the square of the diameter, is even moresensitive to the delay time T_(s) near the optimum value/range, whichillustrates the criticality which may exist in defining the proper delaytime in apparatus 950. According to one embodiment, delay time T_(s) fora given volume of individual drops of UV curable ink may be determinedwith the aim to produce a uniform surface emission and/or to maximizeoverall light output from the illumination system or a designated lightextraction area. This can be done, for example, based on computer-basedoptical modeling, which may include raytracing, or based on actualexperiments in which delay time T_(s) (and, optionally, other printingprocess parameters) may be varied to produce the required surfaceemission uniformity and/or overall light output.

FIG. 39 is a schematic view illustrating an embodiment of a method offorming a pattern of light extraction features 8 on surface 11 of lightguiding sheet 10 using printed UV-curable ink and a material transferfilm. FIG. 39 a ) illustrates a first step in which light-scatteringmesa structures 51 and 52 are formed on surface 11. Mesa structures 51and 52 may be formed by direct printing using non-absorbing lightscattering ink (e.g., commercial UV-curable white ink containing TiO₂particles and forming microdots of relatively low thickness minimizinglight absorption). For example, mesa structure 51 may be formed by asingle drop of the UV ink and mesa structure 52 may be formed by severalindividual drops of the UV ink which are overlapping and/or overprintedon top of one another. Printed mesa structure 51 may be instantly curedupon forming. Printed mesa structure 52 may be instantly cured drop bydrop or layer by layer. Mesa structures 51 and 52 may also includevarious color pigments, dyes or phosphors. According to someembodiments, either one or both mesa structures 51 and 52 may have alayered structure and may be formed by two or more cured ink layersoverprinted on top of each other, e.g., as described above in referenceto FIG. 13 , FIG. 17 and/or FIG. 33 . According to one embodiment,either one or both mesa structures 51 and 52 may have a very thin (e.g.,1-5 micrometers or 5-10 micrometers) bottom layer formed by a color UVink (e.g., red, green, blue, cyan, magenta or yellow ink) or afluorescent UV ink (e.g., containing a phosphor converting blue lightinto red, green or both colors), an intermediate layer of anon-absorbing light scattering ink (which may have a white color whenilluminated by white light), and a protective top layer. The bottomlayer may have one or more sublayers which may be formed, for example,by UV inks having different-color pigments or different-bandgapphosphors. The material of a layer of non-absorbing light scattering inkadjacent to a layer formed by a color or fluorescent ink may includelight scattering particles (e.g., titanium dioxide) having sizesoptimized for maximum light dispersion by diffraction in the respectivecolor. The optimal sizes may be calculated, for example, using theprinciples of Mie theory of light scattering.

FIG. 39 b ) schematically illustrates a step of overprinting mesastructures 51 and 52 using layers 61 and 62, which can be formed, forexample by optically clear UV-curable adhesive ink. The adhesive ink isselected such that it flows well around mesa structure 51 and 52 andadheres strongly to the material of the mesa structures. According toone embodiment, layers 61 and 62 are partially cured using a UV lightsource such that the material of layers 61 and 62 becomes solid butretains some softness and tackiness. According to one embodiment, theadhesive ink may be configured to retain strong adhesiveness even afterbeing fully cured.

According to one embodiment, layers 61 and 62 may be partially curedinstantly, e.g., upon being printed, using a UV lamp attached to therespective printhead and set to provide UV intensity significantly belowwhat is necessary for the full cure. This embodiment may beadvantageously selected to significantly increase the viscosity of theadhesive ink, causing it to gel, and limit the unwanted spread of theadhesive ink under the forces of gravity and/or adhesion/cohesion.According to one embodiment, layers 61 and 62 may be partially cured atonce after the printing process is complete, e.g. by briefly exposingsurface to a wide-area UV light source, as further illustrated in FIG.39 b ). The UV source may also be configured to illuminate surface 12.

FIG. 39 c ) schematically illustrates a step of applying of a selectivematerial transfer film 60 to surface 11. Film 60 has an easy-releasecarrier layer 63 and a transfer layer 64. Carrier layer 63 may beexemplifies by a thin polyester film preferably having a thicknessbetween 10 and 50 micrometers, and even more preferably between 10 and25 micrometers. Transfer layer 64 is formed by a thin layer of materialwhich can be selectively transferred to other surfaces. Film 60 may beconfigured such that the adhesion of transfer layer 64 to carrier layer63 is relatively low, allowing the transfer layer to be easily releasedby applying a slight pull force.

According to some embodiments, transfer layer 64 may be formed byvarious opaque materials, which can be light absorbing (e.g., containingcarbon black or dark pigments at sufficient density) or reflective(e.g., secularly or diffusely reflective). According to someembodiments, transfer layer 64 may be formed by various opticallytransmissive materials, including, for example, a clear or translucentresin impregnated with light scattering particles, color pigments, toner(which can be white or colored), or phosphors. Transfer layer 64 mayalso be formed by various powders (e.g., including color pigment ofphosphor particles) which can be released from carrier layer 63 with arelative ease. Transfer layer 64 may also be configured to be mostlyopaque with some translucency (e.g., allowing some light through) at theselected thickness of the layer.

According to one embodiment, transfer layer 64 is formed by a metallicfoil (e.g., aluminum foil or silver foil) which has high reflectance inthe optical spectrum and a thickness of several micrometers, preferablybetween 2 and 10 micrometers. Non-limiting examples of the materialswhich can be used as selective material transfer film 60 include varioustypes of transfer foils used in digital foiling, e.g., a hot transferfoil or a cold transfer foil. According to one embodiment, transferlayer 64 is formed by a metallic or metallized foil having a smooth andshiny side facing carrier layer 63 and an opposite matte-finish/texturedside which is used to apply film 60 to patterned light guiding sheet 10.

In a further step, selective material transfer film 60 is applied (e.g.,laminated) to surface 11 of light guiding sheet 10. A downward pressurecan be applied to a surface 65 to promote a good physical contact andadhesion of a bottom surface 66 to mesa structures 51 and 52. This canbe done using, for example, a roller, a roll laminator, a platen press,or a vacuum laminator. The downward pressure may be advantageouslyselected to cause partially-cured adhesive layers 61 and 62 to spreadover the respective portions of surface 66, e.g., as schematicallyillustrated in FIG. 39 d ), and ensure a strong bond as well as a goodoptical contact. According to one embodiment, film 60 may be heated(e.g., radiatively or using a heater roller or press plate) to furthersoften the adhesive material of layers 61 and 62 and promote theadhesion.

FIG. 39 d ) further schematically illustrates a step of fully curinglayers 61 and 62 using UV light at full curing power. The respective UVlight source may be provided on the side of surface 12. This may beespecially advantageous when transfer layer 64 and/or carrier film 63are opaque or absorb UV light. Otherwise, the UV light source may alsobe provided on the side of surface 11. According to one embodiment, thisstep may be configured to ensure a permanent bond between film 60 andmesa structures 51 and 52.

FIG. 39 e ) schematically illustrates a step of lifting selectivematerial transfer film 60 off sheet 10 such that portions of transferlayer 64 that are bonded to mesa structures 51 and 52 permanently remainon the mesa structures, selectively forming layers 68 and 69,respectively, and such that the other (non-bonded) portions of transferlayer 64 (portions 66) are removed. Film 60 may be discarded or reusedfor subsequent applications.

As a result of the steps described above, mesa structure 51, layer 61,and layer 68 may cumulatively form one light extraction feature 8 andmesa structure 52, layer 62, and layer 69 cumulatively form anotherlight extraction feature 8. Both light extraction features 8 may have aform of flat-top mesa structures configured to extract light from lightguiding sheet 10. According to one embodiment, additional layers may bedeposited on top of layers 68 and 69. For example, the additional layersmay include a protective clear or opaque lacquer configured to protectthe material of layers 68 and 69 from the ambient air and moisture.

According to one embodiment, layers 68 and 69 may be opaque (e.g.,formed by a metallic foil) and configured to block light emission fromsurface 11 by reflecting the extracted light back towards surface 12,such that light emitting sheet 10 can be configured for one-sidedemission (e.g., for emitting almost all light from surface 12 andemitting virtually no light or a perceptibly small amount of light fromsurface 11) while maintaining transparency at least in the areas locatedbetween mesa structures 51 and 52. Edge-lit sheet 10 withone-sided-emission may be used, for example, as a back light, as a frontlight, e.g., as discussed above in reference to FIG. 13 , as anilluminated sign, or as any other suitable type of wide-areaillumination device. According to one embodiment, layers 68 and 69 maybe optically transmissive and configured to distribute light from bothsides of sheet 10 (e.g., from surfaces 11 and 12) while sheet 10 may beconfigured to maintain transparency at least in the areas locatedbetween mesa structures 51 and 52. A reflective sheet may be provided oneither side of sheet 10, depending on the desired direction of theemission.

According to one embodiment, film 60 can be made sufficiently flexibleand, optionally, elastic, allowing it to stretch longitudinally andlaterally. Furthermore, according to some embodiments, the roller orplate used to press film 60 against surface 11 of sheet 10 can beprovided with a soft layer (e.g., rubber or silicone), such that film 60can conformably wrap around mesa structures 51 and 52 and such thatlayers 68 and 69 can conformably cover the mesa structures, e.g.,similarly to outer layers 778 conformably covering inner layers 777 ofFIG. 13 . Alternatively, a vacuum laminator with a soft and stretchablemembrane may be used for conformably affixing film 60 to mesa structures51 and 52 and enabling the selective transfer of the material of layer64 to surface 11 of sheet 10.

According to one embodiment, the above-described method may be modifiedsuch that mesa structures 51 and 51 and adhesive layers 61 and 62 areformed/printed on surface 66 of transfer layer 64 rather than on surface11 of sheet 10. In order to enhance the adhesion and successfulrelease/formation of layers 68 and 69, optional additional adhesivelayers may be provided between mesa structures 51 and 52 and layer 64.

According to one embodiment, the above-described method may be modifiedsuch that selective material release film 60 is replaced with a diffusereflector film and such that the last (film release) step is notperformed, allowing the diffuse reflector film to remain permanentlyaffixed to sheet 10. The diffuse reflector film may be made highlyflexible and optionally elastic such that the resulting layeredstructure can be flexed together with sheet 10 without the delaminationof the reflector film from sheet 10. In order to enhance affixing thediffuse reflector to sheet 10, a layer of adhesive ink material may befurther printed along one or more edges of sheet 10 and/or one or moreedges of the reflector sheet in the form of bands, such that the diffusereflector can be bonded to sheet 10 along those edges. According to oneembodiment, the step of fully curing adhesive layer 61 may be performedafter releasing film 60 from sheet 10. According to one embodiment, thematerial of adhesive layer 61 may be formulated to retain adhesiveproperties even after the full UV cure, in which case the second curingstep described above can be eliminated.

FIG. 40 schematically illustrates an embodiment of a method of forminglight extraction areas 55 and spacing areas 54 in surface 11 of lightguiding sheet 10 using radiation-curable ink and area fill, according toat least one embodiment of the present invention. The embodiment isexemplified by forming a character “A” which can be illuminated usingLEDs 2 (not shown) optically coupled to sheet 10.

The method of FIG. 40 includes forming a printed outer boundary 791 anda printed inner boundary 792 on surface 11 using a first type of UVcurable ink. The first type of ink may be exemplified, for example, byoptically clear UV ink which has good adhesion to the material of sheet10. The first type of UV curable ink may be printed using a first printhead in a UV printer incorporating multiple print heads. Printedboundaries 791 and 792 define enclosed areas 793 and 794, asschematically illustrated in FIG. 40 a ).

The boundary formation process can be done, for example, by printing therespective outlines of the boundaries on top of each other in multiplepasses using instant UV cure. It may be preferred that each drop of theclear ink is cured instantly upon being deposited onto surface 11.Alternatively, each layer of the ink may be cured at once after printingthe respective outline and before printing the next layer. According todifferent embodiments, each layer may have a total thickness ofapproximately 1 micrometer, between 1 micrometer and 3 micrometers,between 2 micrometers and 5 micrometers, between 4 micrometers and 6micrometers, between 5 micrometers and 10 micrometers, or approximatelyequal to or greater than 10 micrometers. The process can be repeateduntil the desired height of the boundary is formed, e.g., 5-10micrometers, 10-20 micrometers, 20-30 micrometers, 30-50 micrometers, or50-100 micrometers. According to different embodiments, a width ofboundaries 791 and 792 may be selected from one of the following ranges(or any combination thereof): 50-100 μm, 100-200 μm, 200-300 μm, 300-500μm, and 500-1000 μm.

Referring to FIG. 40 b ), area 793 is filled with a second type of ink(e.g., up to the half-height or full-height of boundaries 791 and 792,using a second print head of the same UV printer, preferably withoutrepositioning the substrate (sheet 10), thus forming individual lightextraction patterned area 55 and individual spacing area 54. Accordingto one embodiment, filling area 793 can be done using depositing a largenumber of drops of the second-type ink, so as to form a liquid layerwith the prescribed thickness, with the subsequent curing the layerusing a UV light source, as further schematically illustrated in FIG. 40b ). According to one embodiment, filling area 793 can be done byforming multiple layers of the second-type ink, individually curing eachlayer. According to one embodiment, each individual drop of the UV inkmay be instantly cured. The print head may be configured and operated toprint a two-dimensional pattern/bitmap corresponding to the shape ofarea 793 by scanning across the area in two dimensions while depositingand instantly curing each ink drop. The instant-cure printing processmay be configured to form a continuous fill of area 793. It may also beconfigured to form a partial fill of the area, e.g., by forming a numberof small isolated areas that are free from the ink material. Outerboundary 791 may be configured to prevent or at least limit ink flowoutside the boundaries of the printed character. Inner boundary 792 maybe configured to prevent or at least limit ink flow into area 794.

According to one embodiment, surface 11 in area 793 may be patternedprior to filing with the second type of ink, for example, to enhance theadhesion of the ink to surface 11 and/or enhance the efficiency of lightextraction. The preliminary surface patterning may be particularlyadvantageous when the refractive index of the second type of UV ink islower than that of light guiding sheet 10. In a non-limiting example,when sheet 10 if made from acrylic/PMMA, area 793 may be preliminarypatterned using a CO₂ laser. According to one embodiment, the CO₂ lasermay be configured for forming a dense array of cavities in area 793.According to one embodiment, the CO₂ laser may be configured to etch theentire exposed surface of area 793 and produce a shallow recess with aroughened, matte-finish surface.

According to one embodiment, area 793 may be further overprinted withother types of ink, which may include, for example, clear ink, colorink, color-converting ink, light scattering ink, white-color ink, opaqueink (e.g., metallic ink), and adhesive ink. Multiple layers may beformed using different types of ink in any suitable combination.Furthermore, the method described above in reference to FIG. 40 may alsobe adapted for forming individual light extraction features 8 or anyother light extraction features described in preceding embodiments. Thismethod may also be adapted to form light extraction areas of anysuitable shapes and dimensions. For example, this method may be used toform individual light extraction features 8 with sizes greater than thesize of individual cured microdrops of UV ink, e.g., having dimensionsfrom 100 μm to 10 mm or from 200 μm to 1 mm. According to oneembodiment, the method illustrated in FIG. 40 may be combined with themethod described in reference to FIG. 39 . For example, ink-filled area793 may be overprinted with an adhesive ink. Selective material releasefilm 60 may be subsequently applied to the overprinted area and thenremoved (e.g., for forming a single-side-emission or two-sided-emissionillumination structure having both light emitting andnon-emitting/transparent areas). Alternatively, a diffuse reflector filmmay be affixed to area 793. According to one embodiment, light guidingsheet 10 may have any number of ink-filled areas 793, which can bedistributed over the area of surface 11 according to any desiredone-dimensional or two-dimensional pattern and can be used for formingbacklights or light-emitting indicia of various types.

FIG. 42 schematically depicts an embodiment wide-area light guideillumination system 900 where patterned light extraction area 55 isshaped in the form of a letter “A”. The density of light extractionfeatures 8 increases with a distance from light input edge surface 13and LEDs 2. More specifically, the distance between individual lightextraction features 8 decreases with a distance from light input edgesurface 13 and LEDs 2. In addition, the size of light extractionfeatures 8 increases with a distance from light input edge surface 13and LEDs 2 For example, the density and sizes of light extractionfeatures 8 in zones 1051 and 1052 may be considerably less than thedensity and sizes of light extraction features 8 in zone 1053 such thatthe perceptional brightness of zones 1051, 1052 and 1053 is about thesame. Furthermore, each of zones 1051, 1052 and 1053 may have a densitygradient of respective light extraction features 8 (with the spacingbetween adjacent light extraction features gradually decreasing with adistance from LEDs 2. According to different embodiments, the relativearea cumulatively occupied by light extraction features 8 within zone1053 may be greater than the relative area cumulatively occupied bylight extraction features 8 within each of zones 1051 and 1051 by atleast 1.5 times, at least 2 times, at least 3 times, at least 4 times,at least 5 times, at least 8 times, or at least 10 times.

At least some light extraction features 8 may have layered structures(e.g., similar to those exemplified by light extraction features 602,604, 606, and 608 in FIG. 33 ) configured to cause light emission indifferent colors from different parts of patterned light extraction area55. According to one embodiment, zone 1051 may have light extractionfeatures 8 configured to extract/emit light in a first color (e.g.,red), zone 1052 may have light extraction features 8 configured toextract/emit light in a different second color (e.g., green), and zone1053 may have light extraction features 8 configured to extract/emitlight in a yet different third color (e.g., blue). It is noted that

FIG. 43 schematically depicts an embodiment wide-area light guideillumination system 900 including light guiding sheet 10 (which isilluminated by LEDs 2), a viewable front sheet 80 (cover sheet) and aback sheet 17 sandwiching the light guiding sheet. Front sheet 80 andback sheet 17 are approximately coextensive with light guiding sheet 10and may have about the same length and width dimensions as light guidingsheet 10. Each of light guiding sheet 10, front sheet 80 and back sheet17 has four through holes/openings which are used for fastening therespective sheets together using fasteners 56. Fasteners 56 may beexemplified by metal or plastics rivets or screws. The throughholes/openings may be formed in sheets 10, 80 and 17 using lasercutting, punching or drilling, for example. According to one embodiment,back sheet 17 is formed from a diffusely reflective material such that asurface 103 of back sheet 17 is configured to simultaneously reflect anddeflect light. Suitable non-limiting examples of such materials includebut are not limited to white-color high-brightness, high-density paper,polystyrene, white-painted metal or plastic sheets, and the like.According to one embodiment, surface 103 may be mirrored, e.g., bylaminating a specularly reflective film to sheet 17. According to oneembodiment, sheet 17 may be exemplified by a back-surface mirror (e.g.,glass or plastic mirror). According to one embodiment, front sheet 80 isformed from an optically transmissive material, such as glass, acrylic,polycarbonate, PET, PETG, PVC, and the like. Front sheet 80 includes abackground area 81 and one or more light emitting areas 780. Lightemitting areas 780 are exemplified by letters “A” and “B” in FIG. 43 .

According to one embodiment, background area 81 is formed by an opaqueprint on either front (viewable) or rear surface of sheet 80. The opaqueprint may be formed by a solid fill of the respective front or rearsurface (or both) using a conventional digital printer and usingconventional ink used in signage printing (e.g., UV or solvent-basedink). According to one embodiment, the opaque print is formed by a blackink. According to one embodiment, the opaque print is formed by a colorink and has a uniform, solid fill. According to one embodiment, theopaque print is formed by a color ink and has a non-uniform solid fill,e.g., representing a full-color image print.

According to one embodiment, light emitting areas 780 may be formed byopenings (e.g., non-printed areas) in the opaque print formingbackground area 81. According to one embodiment, light emitting areas780 may be formed by an optically transmissive print selectively formedin respective openings of the opaque print. For example, the letter “A”of FIG. 43 may be printed using a yellow ink or a combination of suchink with a light-scattering ink, the letter “B” may be printed using ared ink or a combination of such ink with a light-scattering ink, andsurrounding background area 81 may be printed using a black ink providedat a thickness sufficient to make the background area 81 substantiallyopaque. The thickness of the printed payers forming light emitting areas780 should be sufficient to provide the desired color of the emittedlight. At the same time, this thickness should be sufficiently low topermit transmitting enough light towards the front side and towards aviewer. According to one embodiment, light emitting areas 780 may be atleast partially transparent or at least translucent, and having avisible light transmittance of at least 50%, at least 60%, at least 70%or at least 80%. According to one embodiment, background area 81 may besubstantially opaque and having an opacity in the visible spectrum of atleast 70%, at least 80% or at least 90% (e.g., having opticaltransmittance of less than 30%, less than 20% or less than 10%).According to an aspect, background area 81 may form a generally opaquemask having optically transmissive openings or optical windows in theareas corresponding to patterned areas 55. Such openings or opticalwindows may form light emitting areas 780.

According to one embodiment, front sheet 80 may be formed by an opaquematerial (e.g., opaque plastic, sheet metal, wood, veneer, cardboard,composite material, etc.) and light emitting areas 780 may be formed byrespectively shaped cutouts in such an opaque material. The uncutportions of the opaque material may form background area 81.

According to an alternative embodiment, front sheet 80 may be formed byan optically translucent or transparent sheet material and backgroundarea 81 may be left intreated (e.g., to preserve the transparency ortranslucent appearance of the sheet material. In addition, according toat least one embodiment, back sheet 17 bay also be formed by anoptically transparent or translucent sheet material which can be thesame or different from the material of front sheet 80.

Light guiding sheet 10 includes a plurality of patterned areas 55separated by separation areas 54. Patterned areas 55 may be patternedfor light extraction using two-dimensional patterns of light extractionfeatures 8 according to the principles discussed above. According to oneembodiment, separation areas 54 may be substantially free from lightextraction features 8. According to an alternative embodiment,separation areas 54 may include light extraction features 8 but at amuch lower density compared to patterned areas 55. According todifferent embodiments an average density or spot density of lightextraction features 8 within patterned areas 55 may be greater than anaverage density or spot density of light extraction features 8 withinseparation areas 54 by at least 10 times, at least 50 times, at least100 times, or at least 1000 times. I was experimentally found thatproviding greater density ratios generally enhances the contrast andperceived visibility of patterned areas 55 and light emitting areas 780for the cases when background area 81 is not completely opaque.

Patterned areas 55 may be configured to represent basically the sameshapes or image content of light emitting areas 780. For example,referring to FIG. 43 , patterned areas 55 have substantially the sameoutlines of the letters “A” and “B” as light emitting areas 780.Furthermore, light emitting areas 780 are disposed in registration withrespect to patterned areas 55 and with a minimum overlap with backgroundarea 81 such that light extracted by patterned areas 55 from lightguiding sheet 10 can be emitted through light emitting areas 780 withoutbeing obscured by the opaque material of background area 81. The throughholes/openings may be used for ensuring the accuratepositioning/alignment of patterned areas 55 and light emitting areas780. It should be noted, however, that, according to at least someembodiments, the relative alignment of light emitting areas 780 andpatterned areas 55 does not have to be very precise to allow for theintended operation of system 900. Furthermore, system 900 may bedesigned to allow for some misalignments and various manufacturingerrors and applicable tolerances. For example, according to oneembodiment, the width of lines forming portions of patterned areas 55(e.g., e.g., the patterned lines or bands forming letters “A” and “B” ofFIG. 43 ) may be made slightly thinner (e.g., by up to 5%, 50-10%,10-20% or 20-30%) than those of matching light emitting areas 780.According to one embodiment, the width of lines forming portions ofpatterned areas 55 (e.g., e.g., the patterned lines or bands formingletters “A” and “B” of FIG. 43 ) may be made slightly wider (e.g., by upto 5%, 50-10%, 10-20% or 20-30%) than those of matching light emittingareas 780, e.g., to ensure that all portions of light emitting areas 780are illuminated regardless of the offset relatively to patterned areas55. According to one embodiment, patterned areas 55 may be shaped anddimensioned to only approximate the outer dimensions or outlines ofrespective light emitting areas 780. It was found that, even though thismay cause some optical losses compared to the case of the preciselymatched shapes and dimensions, the resulting structure may still besignificantly more efficient (e.g., in terms of useful light outputversus input electric power) than patterning the entire area of lightguiding sheet 10. In other words, the embodiments disclosed in referenceto FIG. 43 tend to be much more energy efficient for providing aprescribed brightness for light emitting areas 780 than embodimentsemploying using a uniformly lit backlight (e.g., using fully-patternedlight guiding sheet 10 that emits light from entire area 11).

In operation, LEDs 2 emit light in one or more colors (which can bemixed to create a perceptibly white light) and illuminate light guidingsheet 10 though light input edge surface 13. Light guiding sheet 10mixes the received light beams and guides the light towards patternedlight extraction areas 55. Patterned areas 55 extract light from lightguiding sheet only in the designated areas and emit at least asubstantial portion of the extracted light towards light emitting areas780. Patterned areas 55 may also be configured to emit a portion oflight towards back sheet 17 which recycles that light and reflects itback towards light emitting areas 780. Patterned areas 55 may beconfigured such that a first portion of the recycled light istransmitted through spacing areas between individual light extractionfeatures 8 and a second portion of the recycled light is transmittedthrough light extraction features 8 (which can be made semi-opaque ortranslucent and configured for transmitting light in a directionperpendicular to light guiding sheet 10). Light emitting areas 780 maybe configured to further condition the extracted light (e.g., byangularly redistributing and/or coloring the light) and transmit theextracted light towards a viewing direction. A light ray 113 exemplifiesa light path in wide-area light guide illumination system 900 of FIG. 43. According to different embodiments, system 900 may be configured as anilluminated sign or display. According to one embodiment, it may beconfigured to display illuminated text or linear shapes and graphicsvisually resembling neon signs. Light emitting areas 780 may include oneor more color filtering or color converting layers. For example, lightemitting areas 780 may be printed with an optically transmissive colorink or coated with a color filtering material or phosphor material.

FIG. 44 schematically depicts an embodiment wide-area light guideillumination system 900 which is similar to that of FIG. 43 , exceptthat background area 81 is made transparent and back sheet 17 includesreflective areas 105. Back sheet 17 may be transparent, translucent oropaque and may also contain an image print. Reflective areas 105 havebasically the same or similar shapes as patterned areas 55 and/or lightemitting areas 780 and are disposed in registration with patterned areas55 and/or light emitting areas 780. Reflective areas 105 may be formed,for example, by printing a relatively thick layer of white, highlyreflective ink on surface 103 (or on the opposite surface when backsheet 17 is transparent or translucent. It may also be printed using ametallic ink. Reflective areas 105 may also be formed by cutting therespective shapes from a highly reflective sheet material (e.g., mirrorfilm, reflective vinyl, etc.). According to one embodiment the highlyreflective sheet material is configured to reflect light diffusely (withsignificant light scattering). According to one embodiment the highlyreflective sheet material is configured to reflect light specularly(with the angle of reflection being about the same as the angle ofincidence). The operation is schematically illustrated by the path ofexemplary light ray 113. Reflective areas 105 may be configured toprovide the same basic operation (in terms of light recycling andredirection towards the viewing side) as reflective surface 103 of FIG.43 .

FIG. 45 schematically depicts an embodiment wide-area light guideillumination system 900 in which front sheet 80 is formed into a sleeveat least partially enclosing light guiding sheet 10. Two opposing sidesof front sheet 80 are wrapped/folded about edge surfaces 15 and 16 andform two opposing flaps 83 and 84 and two opposite side edges 85 and 86.Sheet 80 should be dimensioned appropriately to allow forming the sleeve(e.g., its width along the X dimension/coordinate should be greater thanthe respective width dimension of light guiding sheet 10). In thecontext of the present description, the term sleeve may be defined as asheet-form structure which at least partially covers two or morebroad-areas and at least one edge of another sheet-form structure (whichcan be exemplified by light guiding sheet). Suitable examples of sleevesas applied to enclosing light guiding sheet 10 may include sheet-formstructures which completely or partially cover surfaces 10 and 11 aswell as at least one of its edge surfaces 13, 14, 15 and 16. In theexemplary embodiment illustrated in FIG. 45 , the sleeve formed by sheet80 completely covers surface 11 and edge surfaces 15 and 16 and alsopartially covers (at its edges) surface 12. Edge surfaces 13 and 14 areleft open and available for light input from LEDs 2. The sleeve of FIG.45 may be formed, for example, by bending front sheet 80 along therespective lines. When front sheet 80 is made from a plastic material(e.g., acrylic/PMMA, polycarbonate or PETG), the designated bend areasof front sheet 80 may be heated to a softening point (e.g., glasstransitioning temperature) using a strip heater, with the subsequentcooling in the bent state. Once the sleeve is formed, it may be used forpermanently or removably retaining light guiding sheet 10 within thesleeve (e.g., for protecting surface 11 and edge surfaces 15 and 16.Furthermore, according to at least some embodiments, the sleeve may beadvantageously used for retaining the various optical control sheetswhich may be positioned along surfaces 11 and 12 of light guiding sheet10. For example, an optically transmissive light diffusing sheet or afilter (not shown) may be inserted into the sleeve between the frontportion of sheet 80 and surface 11. In a further example, a reflectorsheet or back sheet 17 (not shown) may be inserted into the sleevebetween flaps 83 and 84 and surface 12. In this configuration, it may bepreferred that front sheet 80 is formed from a rigid material andprovided at a sufficient thickness to maintain at least minimal rigidityof the sleeve. At the same time, according to at least one embodiment,the thickness of the material of front sheet 80 may be sufficiently lowto allow for flexing and bending the combined sheet-form structureformed by light guiding sheet 10 (along with the associated diffuser andreflector, if any) and the sleeve-shaped front sheet 80, while retainingits overall integrity and thin, layered structure.

FIG. 46 schematically depicts an embodiment of wide-area light guideillumination system 900 which is similar to that of FIG. 45 but is shownin a different orientation (flipped upside down). The embodiment of FIG.46 further includes through holes formed in light guiding sheet 10 andthe sleeve formed from front sheet 80. It also includes fasteners 56used to permanently fasten the sleeve to light guiding sheet 10. FIG. 46further depicts patterned areas 55 including light extraction features 8(which can be formed in either one or both surfaces 11 and 12). Asexplained in reference to FIG. 43 , patterned areas 55 may be formedsuch that they are disposed in registration with respect to lightemitting areas 780 of front sheet 80 when light guiding sheet 10 andfront sheet 80 are assembled together. Light extraction features 8 maybe formed according to the principles discussed in the foregoingdisclosure such that light is emitted by wide-area light guideillumination system 900 only from light emitting areas 780, which mayrepresent various indicia and individual image objects. By appropriatelyconfiguring discrete light extraction features 8 and light emittingareas 780, each image object may be configured to emit light in adistinct color or may also be configured to emit light in several colors(e.g., different areas of a single image object may emit light indifferent colors).

FIG. 47 schematically depicts an embodiment of wide-area light guideillumination system 900 which is similar to those of FIG. 45 and FIG. 46, except that LEDs 2 illuminate only one edge (edge surface 13) and thesleeve formed by front sheet 80 also includes a flap 87 which iscovering opposing edge surface 14. Light guiding sheet 10 may beremovably inserted into the sleeve through the respective narrowopening. According to one embodiment, front sheet 80 may be providedwith a yet additional flap which may be configured to also cover edgesurface 13 and LEDs 2 such that light guiding sheet 10 can be heldwithin the sleeve while being supported and at least partially coveredfrom all its sides.

FIG. 48 schematically depicts, in a cross-section, an embodiment ofwide-area light guide illumination system 900 and illustrates a sleeveformed around light guiding sheet 10 using front sheet 80 which isfolded at its two opposite edges to create flaps 83 and 84. Light isextracted from light guiding sheet 10 using light extraction features 8and 9 formed in surfaces 12 and 11, respectively. Illumination system900 of FIG. 48 further includes back sheet 17 positioned adjacent tosurface 12 and held in place using flaps 83 and 84. Flaps 83 and 84 maybe shaped appropriately to accommodate back sheet 17 and ease theinsertion of light guiding sheet 10 and back sheet 17 duringillumination system 900 assembly.

FIG. 49 schematically depicts, in a cross-section, an alternativeembodiment of wide-area light guide illumination system 900 in whichflaps 83 and 84 of front sheet 80 are bonded to back sheet 17 using atwo-sided adhesive tape 26 such as a Very High Bond (VHB) tape or thelike. According to one embodiment, flaps 83 and 84 may be bonded to backsheet 17 using other types of adhesives (e.g., cyanoacrylates or UV-cureadhesives) or welded to back sheet 17 (e.g., using heat welding,ultrasonic welding, radio-frequency welding, and the like). Forenhancing the bonding or welding process, back sheet 17 and front sheet80 may be made from similar materials which allow for such bonding orwelding. According to some embodiments both sheets can be made from oneof the following: PET, PETG, PVC, acrylic/PMMA, polycarbonate,polyethylene, polypropylene, and acetate.

Wide-area light guide illumination system 900 may further include one ormore reflective strips 19 positioned near edges of light guiding sheet10 and configured to reflect light emerging from the edges. Eachreflective strip 19 may be bonded to the respective edge using anoptically clear adhesive. Alternatively, it may be attached to edges 85and 86 of the sleeve so as to cover that edge. According to oneembodiment, reflective strip 19 is attached or otherwise positioned tocover edge surface 14 of light guiding sheet 10. According to oneembodiment, reflective strips 19 are attached or otherwise positioned tocover edge surfaces 14, 15 and 16 of light guiding sheet 10. Accordingto one embodiment, reflective strip 19 may be formed from a mirror filmmaterial. According to one embodiment, reflective strip 19 may be formedfrom a reflective light diffusing film material.

FIG. 50 schematically depicts, in a cross-section, an embodiment ofwide-area light guide illumination system 900 in which a sleeveenclosing light guiding sheet 10 is formed by front sheet 80 and backsheet 17 which are bonded together by a two-sided adhesive tape 29.Adhesive tape 29 is provided at a thickness sufficient to accommodatethe combines thickness of light guiding sheet 10 and an optical sheet401 included between light guiding sheet 10 and back sheet 17. Accordingto one embodiment, adhesive tape 29 may be formed from an opticallyclear material. According to one embodiment, adhesive tape 29 may beformed from an opaque material. Optical sheet 401 may be exemplified bya reflector, a lighting diffuser, a prism sheet, a lens array, a colorfilter, or a color converting luminescent film (e.g.,phosphor-containing film). It may also be exemplified by a translucentimage print (e.g., an image printed on a transparent or translucent filmmedia such as those used in backlit illuminated signs) configured fordisplaying images when illuminated from the back. One or more opticalsheets 401 may also be provided between light guiding sheet 10 and frontsheet 80. One or more edge trim channels 450 may be used to cover theexposed edges of the sleeve. Edge trim channels 450 may beconventionally formed from metal or plastic materials. For example, edgetrim channels 450 may be formed from aluminum extrusion, plasticextrusion or single-sided adhesive plastic tape.

FIG. 51 schematically depicts, in a cross-section, an embodiment ofwide-area light guide illumination system 900 in which a sleeveenclosing light guiding sheet 10 is formed by front sheet 80 and backsheet 17 which are joined together at their ends (edges) and bonded orwelded bonded together at those ends (e.g., using adhesive orheat-assisted welding). According to one embodiment, the sleeve may beformed from a heat shrink film. For example, according to a method ofmaking wide-area light guide illumination system 900, an oversized pouchor sleeve may be initially formed from an optically transmissive heatshrinkable material such as polyolefin, polyvinyl chloride (PVC),polytetrafluoroethylene (PTFE), or fluorinated ethylene propylene (FEP).Light guiding sheet 10, one or more optical sheets 401 and optionallyfront sheet 80 and back sheet 17 may be inserted into the oversizedsleeve. Subsequently, the sleeve may be exposed to heat (e.g., runthrough a heat shrink tunnel) to allow its material to shrink and seallight guiding sheet 10 and all additional sheets included into wide-arealight guide illumination system 900. According to an aspect, theembodiments of FIG. 50 and FIG. 51 illustrate configurations of aprotective sleeve which completely encloses light guiding sheet 10(e.g., by enclosing its surfaces 11, 12 and at least edge surfaces 15and 16). It may be appreciated that the sleeve may be similarly formedto also enclose edge surface 14 (not shown) and/or edge surface 13.

FIG. 52 schematically depicts, in a cross-section, an embodiment ofwide-area light guide illumination system 900 in which a linearheat-spreading structural member 109 is fastened to light guiding sheet10, front sheet 80 and back sheet 17 using one or more fasteners 110.According to one embodiment, structural member 109 may be exemplified byan aluminum extrusion channel or angle. Structural member 109 may alsobe exemplified by a channel or angle formed from thin aluminum sheet(e.g., coil). Fasteners 110 may be exemplified by metal or plasticrivets (e.g., blind rivets, tubular/semi-tubular rivets, solid rivets orsplit rivets) or screws. Structural member 109 may include a first leg109′ and a second leg 109″ oriented perpendicular to first leg 109′.First leg 109′ is positioned adjacent to the sheet-form structure formedby light guiding sheet 10, front sheet 80 and back sheet 17. Second leg109″ is positioned in a close proximity to the light input edge of lightguiding sheet 10 and disposed parallel to edge surface 13.

Light guiding sheet 10, front sheet 80 and back sheet 17 may have aseries of through holes distributed along its light input edge (edgesurface 13) in which fasteners 110 can be inserted. In turn structuralmember 109 may likewise have a matching series of through holes formedin first leg 109′, distributed along its longitudinal axis edge andconfigured to accommodate fasteners 110 (e.g., dimensioned toaccommodate the shaft diameter of the respective rivets or screws).

LEDs 2 may be provided on a preferably rigid PCB 140 which can beattached to an inner surface of first leg 109′ using a heat-conductiveadhesive or appropriate fasteners (not shown), such as screws or rivets.Wide-area light guide illumination system 900 may further include asecond linear heat-spreading structural member 108 fastened tostructural member 109 (e.g., using an adhesive tape or mechanicalfasteners. According to some embodiments, structural member 108 may beconfigured as a front cover for the light input portion of thesheet-form structure formed by light guiding sheet 10, front sheet 80and back sheet 17. Structural members 108 and 109 should preferably beopaque and configured to prevent emitting light from illumination system900 towards normal viewing directions. Structural members 108 and 109may be configured to tightly snap together and form a rigid, opaquehousing 1055 substantially enclosing the light input portion ofillumination system 900, including but not limited to edge surface 13,LEDs 2 and PCB 140. The shape of structural members 108 and 109 may beselected to minimize light escape from a linear hollow cavity formed byhousing 1055.

The location of the through holes formed in first leg 109′ shouldpreferably be selected to provide a minimum gap between LEDs 2 and edgesurface 13. According to some embodiments, the gap is less than athickness of light guiding sheet by at least 1.5 times, at least 2times, at least 3 times, at least 4 times, at least 5 times, at least 6times, at least 8 times or at least 10 times. It may be appreciated thatmaintaining a minimum gap between light guiding sheet 10 and LEDs 2 maybe critical for reducing light spillage into the space between lightguiding sheet 10 and structural members 108 and 109. According to oneembodiment, layers of a low-refractive index material may be providedbetween light guiding sheet 10 and outer sheets (e.g., front sheet 80and back sheet 17). According to different embodiments, the extent offirst leg 109′ in a direction transverse to its longitudinal axis isgreater than the respective extent of second leg 109″ and/or structuralmember 108 by at least 1.5 times, at least 2 times, at least 3 times atleast 4 times or at least 5 times, e.g., to provide for enhanced heatdissipation. According to one embodiment, light guiding sheet 10 may bepositioned relatively to LEDs 2 such that at least one of the respectivethrough holes used for attaching sheet 10 to structural member 109 isdisposed between the light emitting apertures of two adjacent LEDs 2. Inother words, the through holes should preferably be offset relatively toLEDs 2 (in a plane of light guiding sheet 10) to minimize interceptinglight by the holes and fasteners 111.

It was experimentally determined that the basic structure of wide-arealight guide illumination system 900 schematically depicted in FIG. 52may be particularly advantageous for thin and flexible configurations ofillumination system 900 and also for the cases where the thickness oflight guiding sheet 10 is comparable to the size of LEDs 2 (e.g., thesheet thickness is about the same as the size of the light emittingaperture of the LEDs or exceeds this aperture by 50% or less). Forexample, fastening light guiding sheet 10 and one or more auxiliarysheets (e.g., front sheet 80 and back sheet 17) to structural member 109using fasteners 111 has shown to help eliminate the slippage of thesheets out of housing 1055 and also prevent shifting light input edgesurface 13 relatively to LEDs 2 when the sheet-form structure protrudingfrom the housing is being flexed or when illumination system 900 issuspended by the housing.

FIG. 53 schematically depicts, in a cross-section, an embodiment ofwide-area light guide illumination system 900 in which light guidingsheet 10 and front sheet 80 are folded at their mid-sections and form alayered sheet structure with two-sided emission (emitting light fromboth exposed sides of the sheet structure). According to one embodiment,light guiding sheet 10 having a thickness from 0.5 mm to 3 mm may befolded using a heated-strip bending machine, producing a folded edge115. Front sheet 80 may have a thickness between 50 m and 1 mm and maybe wrapped around folded edge 115. Reflective strip 19 may be providedin the space between folded edge 115 and front sheet 80 and configuredfor reflecting light that may escape from folded edge 115 back to lightguiding sheet 10. According to one embodiment, reflective strip 19 maybe bonded to an outer surface of folded edge 115. According to oneembodiment, reflective strip 19 may be bonded to an inner surface offront sheet 80. As illustrated in FIG. 53 , light extraction features 8may be formed on both surfaces 11 and 12 of light guiding sheet 10. Backsheet 17 bay be included into the space formed by the fold. According toone embodiment, back sheet 17 may be made from an opaque reflectivematerial and configured to recycle light extracted towards back sheet 17using specular or diffuse reflection. According to one embodiment, backsheet 17 may be made from an optically transmissive material andconfigured to diffuse light extracted towards back sheet 17 as well aspartially reflect and partially transmit that light. According to oneembodiment, back sheet 17 may include an image print illuminated bylight guiding sheet 10. It may be appreciated that the operation oflight guiding sheet 10 is may be particularly vulnerable to the presenceof any surface contaminants which may disrupt TIR and degrade theperformance and/or aesthetics of wide-area light guide illuminationsystem 900. Accordingly, font sheet 80 may be formed from a scratch andsoiling-resistant material and configured to protect light guiding sheet10 and the image print from scratching and soiling. According to anaspect, front sheet 80 forms a sleeve that encloses light guiding sheetand may be configured to provide a similar protection to the sleevesdescribed in preceding embodiments.

According to one embodiment, the edges of light guiding sheet 10corresponding edge surfaces 13 and 14 may be joined such that edgesurfaces 13 and 14 form a single light input edge of the foldedstructure. LEDs 2 may be dimensioned to illuminate both edge surfaces 13and 14. Housing 1055 may be configured to hold the joined edges in placeand at a prescribed distance from LEDs 2 using fasteners 110 or usingother means (e.g., by pressing the edges together and using friction tohold folded light guiding sheet 10, front sheet 80 and back sheet 17 inplace during handling and operation). According to one embodiment, thejoined edges may be bonded together using an optically clear adhesive.According to one embodiment, the respective edges may be separated fromeach other by a small distance using a thin opaque material (e.g., areflective film). Each half of the folded light guiding sheet 10 may beconfigured to emit light independently from each other and may havedifferent light extraction patterns. For example, one half may beconfigured to emit light in a fist color and the other half may beconfigured to emit light a different second color. According to oneembodiment, one half may be configured to display a first illuminatedimage or indicia and the other half may be configured to display adifferent second illuminated image or indicia.

The principles of forming wide-area illumination systems described inforegoing disclosure are not limited to using two-dimensional sheet-formstructures for guiding and distributing light and may be applied withoutlimitations for forming wide-area illumination systems usingone-dimensional or substantially linear structures such as opticalfibers, rods or bars. FIG. 54 schematically depicts, an embodiment of alinear light guide illumination system 910 including LED 2, a lightguiding rod 120 and reflective back sheet 41. Rod 120 is made from ahighly transmissive material such as PMMA, glass or the like. Rod 120has a light input edge 23, an opposing edge 25, a viewable front face72, a pair of side faces 73 and 74 (which may also be viewable) and aback face 75. Each of the faces 72, 73, 74, and 75 includes atwo-dimensional pattern of light extraction features 8. Front face 72has a curved shape in a cross-section. Light extraction features 8 aredistributed along both the longitudinal and lateral extent of face 72,following the curvature of face 72. LED 2 is positioned in a closeproximity to light input edge 23 and is optically coupled to the lightinput edge. A reflective surface may be positioned at edge 25 (andoptionally bonded to it or formed directly on its surface) to reflectunextracted light back into the body of rod 120. Alternatively, anotherLED2 may be optically coupled to edge 25 (e.g., to increase the lightoutput). Light extraction features 8 may be distributed over the areasof the respective faces according to a randomized or orderedtwo-dimensional pattern. The pattern may be arranged to have multiplerows and columns. According to one embodiment, the density of lightextraction features 8 is gradually increasing from light input edge 41to edge 25. According to different embodiments, an area density of lightextraction features at a first location of rod 120 (e.g., near lightinput edge 23) is less than an area density of light extraction featuresat a second location of rod 120 (e.g., at a mid-portion or near edge 25)by at least 2 times, at least 3 times, at least 3 times, at least 4times, at least 5 times, at least 5 times, at least 6 times, at least 8times, at least 10 times, at least 15 times, or at least 20 times.

FIG. 55 schematically depicts, in a cross-section, an embodiment oflinear light guide illumination system 910 in which rod 120 has agenerally rectangular cross-section with faces 73, 74 and 75 beingplanar and face 72 being curved. According to different embodiments,face 72 and any other face rod 120 may be convex or concave. A convexshape may be advantageously selected for one face to provide a lensingeffect for light extraction features 8 disposed on the opposite face. Aconcave shape may be advantageously selected to provide enhanced lightdispersion and to mask light extraction features 8 disposed on theopposite face.

FIG. 56 schematically depicts an embodiment of linear light guideillumination system 910 in which rod 120 has a rectangular outline in across-section, with all of the faces being substantially planar. It isnoted that the relative dimensions of the sides of the rectangularoutline may vary in a broad range, particularly including substantiallyrectangular outlines and highly elongated outlines. For example, rod 120may be shaped in the form of a rectangular bar or strip having a widththat is greater than a height or thickness by at least 1.5 times, atleast 2 times, at least 2.5 times, at least 3 times, at least 4 times,at least 5 times, or at least 10 times. FIG. 57 schematically depicts anembodiment of linear light guide illumination system 910 which issimilar to that of FIG. 56 but in which viewable face 72 has acorrugated surface formed by a series of prisms and furrows. Lightextraction features 8 may be formed on the respective prisms andfurrows. FIG. 58 schematically depicts an embodiment of linear lightguide illumination system 910 in which rod 120 has a roundcross-section. FIG. 59 schematically depicts an embodiment of linearlight guide illumination system 910 in which rod 120 has a half roundcross-section.

FIG. 60 schematically depicts an embodiment of linear light guideillumination system 910 in which round rod 120 is enclosed into anoptically transmissive tubular sleeve 250. Sleeve 250 may be formed froma transparent or translucent material (e.g., FEP, PVC) which shouldpreferably have a lover refractive index than rod 120. Light extractionfeatures 8 may be configured to have a predetermined minimum height(e.g., at least 3 μm, at least 5 μm, at least 8 μm, at least 10 μm, atleast 15 μm, or at least 20 μm) and to help maintain an air gap betweenrod 120 and tubular sleeve 250. For example, it was experimentally foundthat providing such an air gap may reduce TIR suppression in rod 120 andpremature light decoupling from rod 120. The air gap was found to beespecially critical when sleeve 250 is made from medium- tohigh-refractive-index materials (e.g., having n>1.4). According to oneembodiment, sleeve 250 may be formed from a heat shrinkable material.According to one embodiment the material of sleeve 250 may be colored orinclude phosphors and configured to filter or convert the spectrum oflight emitted by LED 2. According to one embodiment, sleeve 250 may beformed from a light diffusing material configured to disperse lightemitted from rod 120 and mask the conspicuity of individual lightextraction features 8 or various irregularities of the pattern of lightextraction features 8.

FIG. 61 schematically depicts an embodiment of linear light guideillumination system 910 in which square rod 120 is enclosed into tubularsleeve 250. According to one embodiment, the rigidity of tubular sleeve250 may be selected such that sleeve 250 contacts rod 120 only at itscorners and that there is always some minimum air gap between the sleeveand faces 72, 73, 74 and 75 of rod 120. This air gap can be madesubstantially greater than the height of individual light extractionfeatures to additionally safeguard the light guiding operation of rod120 from the excessive optical contact of tubular sleeve 250 and thelight guiding surfaces of rod 120.

It is noted that the foregoing embodiments of linear illuminationsystems described upon the case of using solid rods are not limited tothis and may be amenable to employ hollow rods of tubes and pipes madefrom highly transparent materials. Furthermore, possible cross-sectionof light guiding rod 120 are not limited to those depicted or describedin this specification. Further suitable examples of the cross-sectionsand overall configurations of light guiding rods, as well as variouslinear and two-dimensional light guiding structures that can be madeusing such light guiding rods, may be found, for instance, in the '826Patent.

FIG. 62 schematically depicts an embodiment of a wide-area light guideillumination system 920 formed by a planar array of several linear lightguide illumination systems 910 positioned side by side along their sidefaces 73 and 74. A large number of rods 120 may be used to form a wide,planar panel. The panel may also be formed into a curved panel. Forexample, it may be curved along a longitudinal dimension by curvingindividual rods 120 along their longitudinal axis. It may also be curvedalong a lateral dimension, e.g., by offsetting individual rods 120relatively to each other in a transverse plane. Reflective sheet 41located below faces 75 of the array is dimensioned to cover the entireback plane of illumination system 920. LEDs 2 may be exemplified byindividually digitally addressable LEDs configured to emit light invarious brightness and/or color independently from the other LEDs in therespective linear array. In operation, individual LEDs may be energizedselectively causing light emission only from light guiding rods 120which are optically coupled to those LEDs. According to one embodiment,each rod 120 may be configured to confine most of the coupled lightwithin its body and emit that light uniformly along its entire lengthand width using appropriately distributed and configured lightextraction features 8. According to one embodiment, each rod 120 may beconfigured to emit light only in designated areas, e.g., according tothe principles described above in reference to patterned areas 55 ofwide-area light guide illumination systems 900. In other words, thecombined light emitting surface of illumination system 920 formed by thearray of rods 120 may be viewed as a continuous light emitting surface.The emission from this surface may be controlled by the placement andoptical properties of light extraction features 8 and using variousoptical control layers and sheets described in reference to illuminationsystems 900. Additionally, its emission may be controlled byindividually turning on/off or dimming LEDs 2. According to oneembodiment, a digital controller may be provided to supply variablecurrent to LEDs 2 to provide various static illumination effects (e.g.,providing higher light output in certain portions of the light emittingarea than in other portions) of dynamic illumination effects such asflashing or animation. For example, a first group of rods 120 may beilluminated at once using respective LEDs 2 while other rods 120 may bekept in a non-illuminated state. Subsequently, one or more other groupsof rods 120 may be illuminated in a succession, thus creating a “runninglight” dynamic effect. In a further example, illumination system 920 maybe configured to selectively illuminate and/or dim different groups oflight extraction features 8 representing various two-dimensional objectsor indicia (e.g., individual letters or shapes in an illuminated sign),one group at a time, thus creating flashing or animated effects.

FIG. 63 schematically depicts an embodiment of wide-area light guideillumination system 920 in which light guiding rods 120 are spaced apartfrom each other by fixed distances and have different lengths andshapes. Several rods 120 are formed into curved shapes (e.g., usingheat-assisted bending) and disposed in a symmetric arrangementrelatively to central straight rod 120. According to one embodiment, oneor more rods 120 may be formed into one-dimensional shapes (e.g., awave, a circular shape or a free-form) by bending the respective rods inone plane planes. According to one embodiment, one or more rods 120 maybe formed into three-dimensional shapes (e.g., a spiral) by bending therespective rods in two or more planes. Rods 120 are disposed on top ofindividual LEDs 2 distributed with the same spacing/pattern as the lightinput edges of rods 120. Each rod 120 has a two dimensional pattern oflight extraction features 8 formed in one or more faces of the rod.Light extraction features 8 may be distributed over the entire lengthand width of the respective faces and arranged in rows and columns withvariable spacing (e.g., the spacing decreasing with a distance from therespective light input edges). LEDs 2 may be assembled on a common heatsink 1050. Heat sink may have a generally planar configuration includingheat-spreading plate and an array of heat dissipating fins protrudingfrom the plate away from rods 120.

FIG. 64 schematically depicts an embodiment of wide-area light guideillumination system 920 in which light guiding rods 120 and LEDs 2 arearranged into a two-dimensional array and forms a three-dimensionalillumination structure. While rods 120 of FIG. 64 are depicted as beingstraight, it is noted that any or all of rods 120 may also have a 2D or3D curved shape.

Further details of operation of illumination systems shown in thedrawing figures as well as their possible variations will be apparentfrom the foregoing description of preferred embodiments. Although thedescription above contains many details, these should not be construedas limiting the scope of the invention but as merely providingillustrations of some of the presently preferred embodiments of thisinvention. Therefore, it will be appreciated that the scope of thepresent invention fully encompasses other embodiments which may becomeobvious to those skilled in the art, and that reference to an element inthe singular is not intended to mean “one and only one” unlessexplicitly so stated, but rather “one or more.” All structural,chemical, and functional equivalents to the elements of theabove-described preferred embodiment that are known to those of ordinaryskill in the art are expressly incorporated herein by reference and areintended to be encompassed by the present claims. Moreover, it is notnecessary for a device or method to address each and every problemsought to be solved by the present invention, for it to be encompassedby the present claims. Furthermore, no element, component, or methodstep in the present disclosure is intended to be dedicated to the publicregardless of whether the element, component, or method step isexplicitly recited in the claims. No claim element herein is to beconstrued under the provisions of 35 U.S.C. 112, sixth paragraph, unlessthe element is expressly recited using the phrase “means for.”

What is claimed is:
 1. A method of making a wide-area solid-stateillumination panel, comprising: providing a back sheet; providing afront sheet approximately coextensive in size with the front sheet;providing a wide-area light emitting panel comprising one or moresolid-state light sources and having al light emitting area which isless than a total area of the front sheet; positioning the wide-arealight emitting panel between the front and back sheets; providing one ormore strips of a double sided adhesive tape having a thickness which isapproximately equal to or greater than a total thickness of a lightemitting portion of the wide-area light emitting panel; and joining asurface of the front sheet with a surface of the back sheet along one ormore perimeter edges using the one or more strips of the double sidedadhesive tape so as to form a thin and hollow sheet-form structurehaving a generally uniform thickness and at least partially enclosingthe wide-area light emitting panel.
 2. The method of making a wide-areasolid-state illumination panel as recited in claim 1, wherein the thinand hollow sheet-form structure is adapted for being flexed whileretaining the generally uniform thickness.
 3. The method of making awide-area solid-state illumination panel as recited in claim 1, whereinthe thin and hollow sheet-form structure is adapted for being operablyretained in a bent or flexed configuration.
 4. The method of making awide-area solid-state illumination panel as recited in claim 1, whereinthe wide-area light emitting panel comprises a two-dimensional patternof optical structures configured to distribute light emitted by the oneor more solid-state light sources.
 5. The method of making a wide-areasolid-state illumination panel as recited in claim 1, wherein thewide-area light emitting panel comprises a two-dimensional pattern ofdiscrete optical structures configured to distribute light emitted bythe one or more solid-state light sources, wherein the two-dimensionalpattern has a plurality of first areas having a greater density of theoptical structures and one or more second areas having a lower densityof the optical structures and separating at least two of the pluralityof first areas from one another.
 6. The method of making a wide-areasolid-state illumination panel as recited in claim 1, wherein thewide-area light emitting panel comprises a two-dimensional pattern ofdiscrete optical structures configured to distribute light emitted bythe one or more solid-state light sources, wherein the two-dimensionalpattern has a plurality of first areas having a lower density of theoptical structures and one or more second areas having a greater densityof the optical structures and separating at least two of the pluralityof first areas from one another.
 7. The method of making a wide-areasolid-state illumination panel as recited in claim 1, wherein the frontsheet is formed from an optically transmissive material and has one ormore optically transmissive areas and one or more opaque areas.
 8. Themethod of making a wide-area solid-state illumination panel as recitedin claim 7, wherein one of the one or more optically transmissive areascomprises a printed layer of ink having a first color and another one ofthe one or more optically transmissive areas comprises a printed layerof ink having a second color which is different than the first color. 9.The method of making a wide-area solid-state illumination panel asrecited in claim 7, wherein the one or more opaque areas are formed by aprinted layer of an opaque ink.
 10. The method of making a wide-areasolid-state illumination panel as recited in claim 1, wherein thewide-area light emitting panel comprises a two-dimensional pattern ofoptical structures configured to distribute light emitted by the one ormore solid-state light sources, wherein the front sheet is formed froman opaque material and has one or more cutouts disposed in registrationwith one or more optical structures of the two-dimensional pattern. 11.The method of making a wide-area solid-state illumination panel asrecited in claim 1, wherein the double sided adhesive tape is formedfrom an optically transmissive material.
 12. The method of making awide-area solid-state illumination panel as recited in claim 1, whereinthe double sided adhesive tape is formed from an opaque material. 13.The method of making a wide-area solid-state illumination panel asrecited in claim 1, further comprising positioning a first layer of anopaque light-blocking material on a first side of the wide-area lightemitting panel so as to cover an area associated with the one or moresolid-state light sources from the first side, and positioning a secondlayer of an opaque light-blocking material on an opposite second side ofthe wide-area light emitting panel so as to cover the area associatedwith the one or more solid-state light sources from the second side. 14.The method of making a wide-area solid-state illumination panel asrecited in claim 1, wherein the wide-area light emitting panel comprisesan optical waveguide having a thickness from 0.5 mm to 3 mm andcomprising a two-dimensional pattern of light extraction structures,wherein the front sheet has one or more substantially opaque areas andone or more optically transmissive areas, wherein the two-dimensionalpattern of light extraction structures comprises one or more areashaving a greater density of the light extraction structures and one ormore areas having a lower density of the light extraction structures,wherein the at least one of the one or more areas having a greaterdensity of the light extraction structures is disposed in registrationwith the one or more optically transmissive areas and at least one ofthe one or more area having a lower density of the light extractionstructures is disposed in registration with the one or moresubstantially opaque areas.
 15. The method of making a wide-areasolid-state illumination panel as recited in claim 1, wherein thewide-area light emitting panel comprises an optical waveguide having athickness from 0.5 mm to 3 mm and comprising a two-dimensional patternof light extraction structures, wherein the front sheet has one or moreareas having a lower optical transmittance and one or more areas havinga greater optically transmittance, wherein the two-dimensional patternof light extraction structures comprises one or more areas having agreater density of the light extraction structures and one or more areashaving a lower density of the light extraction structures, wherein theat least one of the one or more areas having a greater density of thelight extraction structures is disposed in registration with the one ormore areas having a greater optically transmittance and at least one ofthe one or more area having a lower density of the light extractionstructures is disposed in registration with the one or more areas havinga lower optically transmittance.
 16. The method of making a wide-areasolid-state illumination panel as recited in claim 1, wherein at least aportion of the front or back sheet is formed from a metallic materialand has a bent or folded section.
 17. The method of making a wide-areasolid-state illumination panel as recited in claim 1, wherein thewide-area light emitting panel comprises a channel extending parallel toan edge of the front sheet and at least partially enclosing the one ormore solid-state light sources.
 18. The method of making a wide-areasolid-state illumination panel as recited in claim 1, comprisingpositioning a layer of a light blocking material along a perimeter edgeof the front sheet.
 19. A method of making a wide-area solid-stateillumination panel, comprising: providing a back sheet; providing afront sheet approximately coextensive in size with the front sheet;providing a wide-area light emitting panel comprising one or moresolid-state light sources and having al light emitting area which isless than a total area of the front sheet; positioning the wide-arealight emitting panel between the front and back sheets; providing anadhesive material at a thickness that is approximately equal to orgreater than a total thickness of a light emitting portion of thewide-area light emitting panel; and joining a surface of the front sheetwith a surface of the back sheet along one or more perimeter edges usingthe adhesive material, forming a thin and hollow sheet-form structurewhich has a generally uniform thickness and at least partially enclosesthe wide-area light emitting panel.
 20. A method of making a wide-areasolid-state illumination panel, comprising: providing a back sheet and afront sheet which are approximately coextensive in size; providing awide-area light emitting panel comprising one or more solid-state lightsources and having al light emitting area which is less than a totalarea of the front sheet; positioning the front and back sheets parallelto one another at a distance of 0.5 mm to 3 mm; positioning thewide-area light emitting panel between the front and back sheets; andjoining a back surface of the front sheet with a front surface of theback sheet along two or more perimeter edges using an adhesive material,at least partially enclosing the wide-area light emitting panel.